The following citations provide some very noteworthy commentaries of the last several decades, if not centuries, concerning this age-old question.
The most salient answers appear to be related to the grace of God to the (unpentitent) sinner and/or providing to others the testimony of sin and its ultimate consequence (that is, final exclusion from God's presence).
Although Cain expressed not penitence, but fear of punishment, God displayed His long-suffering and gave him the promise, “Therefore (לָכֵן not in the sense of לֹא כֵן, but because it was the case, and there was reason for his complaint) whosoever slayeth Cain, vengeance shall be taken on him sevenfold.” כָּל־הֹרֵג קַיִן, is cas. absolut. as in Gen. 9:6; and הֻקַּם avenged, i.e., resented, punished, as Ex. 21:20, 21. The mark which God put upon Cain is not to be regarded as a mark upon his body, as the Rabbins and others supposed, but as a certain sign which protected him from vengeance, though of what kind it is impossible to determine. God granted him continuance of life, not because banishment from the place of God’s presence was the greatest possible punishment, or because the preservation of the human race required at that time that the lives of individuals should be spared,—for God afterwards destroyed the whole human race, with the exception of one family,—but partly because the tares were to grow with the wheat, and sin develop itself to its utmost extent, partly also because from the very first God determined to take punishment into His own hands, and protect human life from the passion and wilfulness of human vengeance.
Keil, C. F., & Delitzsch, F. (1996). Commentary on the Old Testament (Vol. 1). Peabody, MA: Hendrickson, 72.
Why does God preserve the life of this murderer? This is particularly perplexing since Torah requires capital punishment for murder. No substitute for this crime, such as monetary penalty, was acceptable (e.g., Num 35:32). Perhaps the answer is that by the “sign” God prevents the spread of bloodshed that otherwise would escalate. Moreover, God is declaring that life and death are his prerogative, which he does not share with anyone except by divine sanction (cp. 9:5–6). God’s judgment against the culprit is restrained by his grace. His promise of procreation is not thwarted even by human murder (1:28; 3:15, 20). Cain will live outside “the LORD’S presence,” which is another narrative reminder of Adam’s crime and penalty (3:22). This same expression also describes Jonah, who fled the Lord’s presence (Jonah 1:3, 10). In the Mosaic community “the LORD’S presence” often referred to the sacred tabernacle (e.g., Lev 9:24; 22:3; Num 20:9; 1 Sam 21:7). Routinely, the garden tōlĕdōt (2:4–4:26) has employed the same language and imagery associated with the tabernacle. Here the setting is reminiscent of biblical excommunication, requiring death (e.g., Exod 31:14; Lev 18:29) or quarantine (e.g., Lev 13:46; 15:31). Cain’s residing in “the land of Nod, east of Eden,” implies that he is further removed from the garden than Adam. “Nod” is a play on the word nād, meaning “wanderer,” which refers to the sentence against Cain in 4:11–12, 14. Scripture does not speak again of “Nod,” and no specific locale is known. It may be that Nod is simply meant to say that wherever Cain sojourned could be called the “land of the Wanderer.”
Mathews, K. A. (1996). Genesis 1-11:26 (Vol. 1A). Nashville: Broadman & Holman Publishers, 278-279.
But why this exemption of Cain? To this question every kind of answer has been given (comp. Delitzsch and Keil). The chief thing was, that this banishment had in itself the significance of a social human death. It was a member cut off from the human community, as in the New Testament history of Judas. Besides, the unfolding of the Cainitish existence was to reveal an unfolding of death in a higher degree, and, at the same time, to do service to human culture in the dissemination of the Cainitish talent. Finally, there comes into consideration, in relation to Cain, what is said by Delitzsch: “He was gracious to him in the prolongation of his time of grace, because he recognized the sin as sin.” But at the same time, God himself gives here the first example for the significance of the law of pardon in the later society. To demand the death of Cain was properly the right only of Abel’s parents. But these were also Cain’s parents. The right of pardoning is the right of modifying or mitigating the punishment in view of special mitigating circumstances.
Lange, J. P., Schaff, P., Lewis, T., & Gosman, A. (2008). A Commentary on the Holy Scriptures: Genesis. Bellingham, WA: Logos Bible Software, 260.
As to why this special privilege was granted to Cain, it was not because “the early death of the pious Abel was in reality no punishment, but the highest boon” (Kalisch), nor because banishment from God’s presence was the greatest possible punishment, “having in itself the significance of a social human death” (Lange), nor because it was needful to spare life for the increase of posterity (Rosenmüller); but perhaps — 1. To show that “Vengeance is mine; I will repay, saith the Lord.” 2. To prove the riches of the Divine clemency to sinful men. 3. To serve as a warning against the crime of murder. To this probably there is a reference in the concluding clause. . . . And dwelt in the land of Nod. The geographical situation of Nod (Knobel, China?) cannot be determined further than that it was on the east of Eden, and its name, Nod, or wandering (cf. vers. 12, 14; Ps. 56:8), was clearly derived from Cain’s fugitive and vagabond life (vide Michaelis, ‘Suppl.,’ p. 1612; and cf. Fürst, ‘Lex.,’ sub voce), “which showeth, as Josephus well conjectureth, that Cain was not amended by his punishment, but waxed worse and worse, giving himself to rapine, robbery, oppression, deceit” (Willet).
Spence-Jones, H. D. M. (Ed.). (1909). Genesis. London; New York: Funk & Wagnalls Company, 82.
By all estimates God’s mark, whatever it was, was an amazing grace. Cain was cursed and separated from God, yet guarded by God. Cain’s life still belonged to God. He bore God’s image, however disfigured that image was. This was the utmost mercy that God could do, and does, for the unrepentant. “Then Cain went away from the presence of the LORD and settled in the land of Nod, east of Eden” (v. 16). There is astounding grace right here in one of the darkest scenes in Genesis. Observe that the Lord did not abandon guilty Cain. When Cain arrogantly brought his sparse offering to God, and God saw his evil anger, God did not turn away from him. That is grace. God, in fact, engaged Cain in a fatherly manner with probing, remedial questions. God did not leave him exposed to Satan without recourse. Such grace. God then exhorted Cain to withstand temptation. Again, grace. After the murder, the Lord listened to Cain’s unrepentant, self-pitying plea. Finally, God placed a sign upon Cain that protected him for the remainder of his natural life. Amazing grace! Did Cain repent? Probably not. The New Testament Scriptures uniformly speak of Cain in the negative with phrases like “the way of Cain” (Jude 11) and one “who was of the evil one and murdered his brother” (1 John 3:12).
Hughes, R. K. (2004). Genesis: Beginning and Blessing. Wheaton, IL: Crossway Books, 107.
II. Here is God’s confirmation of the sentence; for when he judges he will overcome, v. 15. Observe, 1. How Cain is protected in wrath by this declaration, notified, we may suppose, to all that little world which was then in being: Whosoever slayeth Cain, vengeance shall be taken on him seven-fold, because thereby the sentence he was under (that he should be a fugitive and a vagabond) would be defeated. Condemned prisoners are under the special protection of the law; those that are appointed sacrifices to public justice must not be sacrificed to private revenge. God having said in Cain’s case, Vengeance is mine, I will repay, it would have been a daring usurpation for any man to take the sword out of God’s hand, a contempt put upon an express declaration of God’s mind, and therefore avenged seven-fold. Note, God has wise and holy ends in protecting and prolonging the lives even of very wicked men. God deals with some according to that prayer, Slay them not, lest my people forget; scatter them by thy power, Ps. 59:11. Had Cain been slain immediately, he would have been forgotten (Eccl. 8:10); but now he lives a more fearful and lasting monument of God’s justice, hanged in chains, as it were. 2. How he is marked in wrath: The Lord set a mark upon Cain, to distinguish him from the rest of mankind and to notify that he was the man that murdered his brother, whom nobody must hurt, but every body must hoot at. God stigmatized him (as some malefactors are burnt in the cheek), and put upon him such a visible and indelible mark of infamy and disgrace as would make all wise people shun him, so that he could not be otherwise than a fugitive and a vagabond, and the off-scouring of all things.
Henry, M. (1994). Matthew Henry’s Commentary on the Whole Bible: Complete and Unabridged in One Volume. Peabody: Hendrickson, 19.
God said that he would prevent anyone from killing Cain with impunity. God did this not because he was granting the murderer a favor, but for the sake of posterity, in order to preserve human life. The Lord declared that if anyone imitated Cain, they would be punished even more severely.
Calvin, J. (2001). Genesis. Wheaton, IL: Crossway Books, 59.
“vengeance will be taken on him sevenfold” The term “sevenfold” seems to mean complete vengeance (BDB 988). Apparently God left Cain alive as an even more poignant sign of sin. The rabbis say that God will take vengeance on him in seven generations which would be Lamech. There is a rabbinical legend that verse 23 refers to Lamech and his son, Tubal-Cain, killing Cain by accident. - “appointed a sign for Cain” This was either a sign (BDB 16, “a mark”) of (1) God’s mercy amidst judgment or (2) God’s sustaining His judgment through time. The rabbis say that God put an animal horn in the midst of Cain’s head. However, it seems more probable that it was a mark on the forehead (cf. Ezek. 9:4, 6).
Utley, R. J. (2001). How it All Began: Genesis 1–11 (Vol. Vol. 1A). Marshall, Texas: Bible Lessons International, 75.
Even though Cain’s fear came from a skewed sense of justice, God did not overlook it, so He put a mark on Cain (4:15). This was not likely a mark that Cain had to wear visibly, for that would only have increased his chances of being targeted by an enemy. It may indicate that God’s promise was a sign of His protection to give Cain peace of mind, in that God was his assurance of safe conduct. What an expression of love for the unrepentant sinner! As ever, God wanted to forgive and redeem Cain, but Cain was more concerned with the level of his punishment than forgiveness for his sin. In later biblical law, he would have received the death penalty. That he did not is evidence of God’s abundant grace. No indication of the nature of the sign is recorded, but its purpose is very clear: so that no one who found him would kill him (4:15). As to the location of Cain’s new home, it was east of Eden. Perhaps the land of Nod was not so much a place as it was a lifestyle, one of nomadic wandering.
Williams, W. G. (1999). Genesis: A Commentary for Bible Students. Indianapolis, IN: Wesleyan Publishing House, 82-83.
If any one slays Cain, etc.] It is forbidden to slay Cain; it is prohibited to take blood revenge. Here the Torah expresses its opposition to blood-revenge, which was practised in the ancient East. Only the Lord, the Judge of the whole earth, and human judges who judge in His name, are permitted to pass sentence on the murderer; not the relations of the murdered man in the heat of their anger. See my remarks on this subject in the introduction, § 5, pp. 184 f. The verse says, if any one slays CAIN, not if anyone slays YOU, because this is a proclamation addressed to all mankind. The Lord informs Cain (Then the Lord said to HIM) of His decree and His proclamation. The words כָּל־הֹרֵג קַיִן kol-hōrēgh Qayin [literally, ‘any one slaying Cain’; rendered: if any one slays Cain] are not the subject of יֻקָּם yuqqām [which in that case would mean: ‘vengeance shall be taken on him’], but constitute a separate, dependent clause [casus pendens], signifying: if any one will slay Cain, then Cain shall be avenged sevenfold. Compare, for example, 1 Sam. 2:13: when any man offered sacrifices [literally, ‘every man sacrificing a sacrifice’], the priest’s servant would come, etc. (Gesenius—Kautzsch2, § 116 w.).
Cassuto, U. (1998). A Commentary on the Book of Genesis: Part I, From Adam to Noah (Genesis I–VI 8). (I. Abrahams, Trans.). Jerusalem: The Magnes Press, The Hebrew University, 225-226.
‘And the Lord set a mark upon Cain.’ What the opinion of others is about this mark, I know not; to me it seems like those in Timothy, who had ‘their conscience seared with a hot iron’ (1 Tim 4:2). Which words are an allusion to the way of the magistrates in their dealing with rogues and felons; who that they may be known to all, are either in the hand, shoulder, or cheek branded with a hot iron. So Cain was marked of God for a reprobate, for one that had murdered a righteous man, even of envy to the goodness of his work: But the mark (as it was on those in Timothy) was not on any outward or visible part of his body, but (as there the apostle expresseth it) even upon his very conscience; his conscience then had received the fire-mark of the wrath and displeasure of God, which, as a burning iron doth to the flesh, had left such deep impression therein, that it abode as a scar or brand upon him, in token that good would for ever after hold him for a fugitive rogue or vagabond. ‘And the Lord set a mark upon Cain, lest any finding him should kill him.’ For though the mark was branded with burning upon his conscience, and so inward and invisible; yet the effects of this hot iron might be visible, and seen of all: the effects, I say, which were, or might be, his restlessness in every place, his dejectedness, the sudden and fearful pangs and agonies of his mind, which might break out into dolorous and amazing complaints; besides, his timorous carriage before all he met, lest they should kill him; gave all to understand, that God had with a vengeance branded him. And indeed this was such a mark as was amazing to all that beheld him, and did ten times more make them afraid of spilling blood, than if any visible mark had been set upon him; of for by his trouble and distress of mind, they saw, what was the guilt of blood: and by his continual fear and trembling under the judgment of God, what it was to be in fear of, nay, to have the first fruits of everlasting damnation. Thus therefore God reserved Cain to the judgment which he had appointed for him.
Bunyan, J. (2006). An Exposition of the First Ten Chapters of Genesis (Vol. 2). Bellingham, WA: Logos Bible Software, 451.
Cain stays ostracised but his life is spared, “and therefore whosoever slays Cain vengeance shall be taken on him sevenfold” (Genesis 4:15). The quality of God’s mercy is hinted at here, but not the quality of Man’s repentance. We are not told that Cain was penitent and regretted his wrongdoing, and thus God’s action here is not in response to penitence but is simply a softening of the punishment by the exercise of mercy. One could have interpreted Cain’s statement “my sin is too great to bear” (verse 13) as stemming from a heavy sense of guilt, but the sequence in verse 14 shows clearly that he is referring to his punishment and not to his sin. In other stories of the Bible, as in the story of David and Bath-sheba (2 Samuel 11–12) and in that of Ahab and Naboth (1 Kings 21), penitence follows the admission of guilt and God’s pardon comes in its wake. These elements do not appear in our context. The final act of the story of Cain and Abel is God’s mercy and compassion. The God who admonishes and punishes is also the God who forgives, and he is the same God who did not deign to accept Cain’s offering, and we know not why.
Adar, Z. (1990). The Book of Genesis: An Introduction to the Biblical World. Jerusalem: The Magnes Press, The Hebrew University, 30-31.
Therefore, the Lord declares, if any will imitate Cain, not only shall they have no excuse in his example, but shall be more grievously tormented; because they ought, in his person, to perceive how detestable is their wickedness in the sight of God. Wherefore, they are greatly deceived who suppose that the anger of God is mitigated when men can plead custom as an excuse for sinning; whereas, it is from that cause the more inflamed. And the Lord set a mark. I have lately said, that nothing was granted to Cain for the sake of favouring him; but for the sake of opposing, in future, cruelty and unjust violence. And, therefore, Moses now says, that a mark was set upon Cain, which should strike terror into all; because they might see, as in a mirror, the tremendous judgment of God against bloody men. As Scripture does not describe what kind of mark it was, commentators have conjectured, that his body became tremulous. It may suffice for us, that there was some visible token which should repress in the spectators the desire and the audacity to inflict injury.
Calvin, J., & King, J. (2010). Commentary on the First Book of Moses Called Genesis (Vol. 1). Bellingham, WA: Logos Bible Software, 214.
Therefore] i.e. on account of Cain’s entreaty, Jehovah’s mercy is shewn to the first murderer. Cain has no friend: Jehovah, by an act of benevolence and authority, will protect him, and undertake his cause even in the desert.
A slight variation in text accounts for LXX οὐχ οὕτως, Lat. Nequa-quam ita fiet.
vengeance...sevenfold] i.e. if Cain were killed, seven deaths would be exacted in retaliation; the murderer and six of his family would forfeit their lives, cf. 2 Sam. 21:8. The words of Jehovah are noticeable, because (1) they emphasize the corporate responsibility of family life, which so often meets us in the O.T.; and (2) they recognize, but regulate, blood-revenge, as a disciplinary primaeval custom of Semitic life. This Oriental custom, while recognized in the O.T. as part of Israelite institutions, is continually being restricted by the operation of the spirit of love, gradually revealed by prophet and by law, in the religion of Jehovah.
the LORD appointed a sign for Cain] The popular expression “the brand of Cain,” in the sense of “the sign of a murderer,” arises from a complete misunderstanding of this passage. The object of the sign was to protect Cain. It was a warning that should prevent the avenger of blood from slaying him. Even in the desert Jehovah would be Cain’s champion. We have no means of knowing what the sign was. The words imply that some visible mark, or badge, was set upon Cain’s person. If so, it may have some analogy to the totem mark of savage tribes. “There seems little doubt, that the sign which Jahveh gave to Cain … was a tattoo mark, probably on his forehead (cf. Ezek. 9:4, 6), to show all men that Cain was under His protection, and thus to save his life. In all probability the mark was the ‘sign of Jahveh,’ the tav (Ezek. 9:4, 6)—which was once doubtless worn quite openly by His devotees, and only afterwards degenerated into a superstition.” (Gordon, Early Traditions of Genesis, p. 211.)
Ryle, H. E. (1921). The Book of Genesis in the Revised Version with Introduction and Notes. Cambridge: Cambridge University Press, 77.
The manner in which God responds to Cain is of special interest. The initial Hebrew lakhen, here rendered “I promise,” frequently introduces a solemn declaration, while the formulation of the reassurance derives from the realm of law. The unusually emphatic language is directed first to Cain, in order to allay his mortal fear, and then to the world at large, as a kind of royal proclamation that has the force of law. It states that despite his crime, Cain still remains under God’s care.
sevenfold Saadiah, Bekhor Shor, and Radak take this simply as a figure of speech meaning “abundantly” or “severely.” Others take the number literally, meaning that seven of the assailant’s family would be killed or that vengeance would continue to the seventh generation.
vengeance This is one of the few passages in which the biblical Hebrew stem n-k-m has its primitive meaning of exacting revenge. Otherwise, it has the sense of redressing the imbalance of justice.
a mark This phrase has been persistently misunderstood. The reference is not to a stigma of infamy but to a sign indicating that the bearer is under divine protection. Hebrew ʾot here probably involves some external physical mark, perhaps on the forehead, as in Ezekiel 9:4–6, serving the same function as the blood of the paschal lamb smeared on the lintels and doorposts of each Israelite house in Egypt. It is also possible, though less likely, that the “sign” consists of some occurrence that serves to authenticate the divine promise as being inviolable. In that case, the text would be rendered: “The LORD gave Cain a [confirmatory] sign that no one who met him would kill him.”
Sarna, N. M. (1989). Genesis. Philadelphia: Jewish Publication Society, 35.