Before an answer is given to your question here are a few things to bring up for your consideration.
Jehovah saith unto Cain, ‘Why hast thou displeasure? and why hath thy countenance fallen? 7Is there not, if thou dost well, acceptance? and if thou dost not well, at the opening a sin-offering is crouching, and unto thee its desire, and thou rulest over it.’
First Iook at the word hattat Which quite often is translated as sin .
It also means sin offering, it changes the context completely different from many translations that say sin is crouching at the door.
It's possible that that word ḥaṭ·ṭāṯ quite often mean sin offering could be so in this passage of scripture as well.
◄ ḥaṭ·ṭāṯ ►
ḥaṭ·ṭāṯ — 52 Occurrences
HEB: תֵיטִ֔יב לַפֶּ֖תַח חַטָּ֣את רֹבֵ֑ץ וְאֵלֶ֙יךָ֙
HEB: מִח֖וּץ לַֽמַּחֲנֶ֑ה חַטָּ֖את הֽוּא׃
NAS: the camp; it is a sin offering.
KJV: the camp: it [is] a sin offering.
INT: without the camp sin he
HEB: וּפַ֨ר חַטָּ֜את תַּעֲשֶׂ֤ה לַיּוֹם֙
NAS: a bull as a sin offering for atonement,
KJV: a bullock [for] a sin offering for atonement:
INT: A bull A sin shall offer day
HEB: בַּשָּׁנָ֑ה מִדַּ֞ם חַטַּ֣את הַכִּפֻּרִ֗ים אַחַ֤ת
NAS: on it with the blood of the sin offering of atonement
KJV: with the blood of the sin offering of atonements:
INT: A year the blood of the sin of atonement onc
With that in mind here's another possible interpretation of that scripture.
if you do well [believing Me and doing what is acceptable and pleasing to Me], will you not be accepted?
What made Able accepted was his offering from his flock. It was an animal from one of his flocks.
Of course we know Cain's offering was not accepted because it was from the ground and the Lord tells him how he would be accepted.
Is there not, if you do well, acceptance? And if you do not do well, sin [[or a sin-offering]] is lying at the opening, and its [[or His]] desire [is] for you, and you rule over it [[or by Him]].” Literal standard version
Is there not, if thou dost well, acceptance? and if thou dost not well, at the opening a sin-offering is crouching, and unto thee its desire, and thou rulest over it.' YLT
This sin offering was desiring Cain to take it, it was alive and lying down at the door, it is there given its life on behalf of Cain and there it was provided for him right at his door. In fact it was lying right there waiting for Cain to rule over it by taking its life and offer that to the Lord and he would be accepted and have done well.
Most translation say it was crouching but take another look at what that word means.
- rabats ►
rabats: stretch oneself out, lie down, lie stretched out
Original Word: רָבַץ
Part of Speech: Verb
Phonetic Spelling: (raw-bats')
Definition: stretch oneself out, lie down, lie stretched ou
This is an offering is not crouching but it's actually lying down stretched out.
This could've actually been a sin offering at the door.,
◄ ḥaṭ·ṭāṯ ►
Quite often this word is translated as an sin offering which totally makes sense. The Lord wanted an offering that was the only way to do well. It was not to master sin.
Able did not master sin, he killed one of the lives from his flock and offered it to the Lord as a sin offering.
It was an offering the Lord wanted and that would mean he had done well. It looks like that there was a sin offering lying down for Cain to kill right at his door ready for Cain to rule over its life by killing it. He could then offer that to the Lord as his sin offering, pleasing the Lord and therefore
be accepted because he had done well.
Cain evidently did not want to slay this offering that was lying at his door but was willing to slay his very own brother instead.
We see the mercy of the Lord manifested to Cain when he was going to have to leave the Lord's presence and that was more than he could bear.
The blood that was spilled on the ground from Able's blood is crying out to the Lord. We know That the Lord's blood that he shed was speaking something different than Ables.
Jesus and to blood of sprinkling, speaking better things than that of Abel! Heb. 12:24
Already we see a foretelling of forgiveness that will be provided for the sins of the world by the sacrifice of the Lamb of God.
One of Cain's judgment from the Lord was that the earths would no longer provide for him. He too is learning everything comes from the hand of God. We also see God's forbearance on him with the judgments, just like we saw the forbearance and love that was provided for his parents Adam and Eve when they sinned. He had provided a covering for them.
He also provided a covering for Cain by marking him so others would not kill him. Oh the mercy of the Lord.
Now onto your question.
“Then the Lord said to him, “Not so! If anyone kills Cain, vengeance shall be taken on him sevenfold.” And the Lord put a mark on Cain, lest any who found him should attack him.” Genesis 4:15
Why wouldn’t God demand an eye for an eye, as He would later? Why is Cain being spared?
First and foremost we are seeing God's mercy being displayed and remember he had provided a lamb before he even created man. The mercy of God can only be seen in light of those who need mercy.
We see the judgments that the Lord does give Cain and since he gave him those judgments he is just.
There was no law given until after the flood. An eye for an eye had not yet been implemented.
And surely I will require the life of any man or beast by whose hand your lifeblood is shed. I will demand an accounting from anyone who takes the life of his fellow man:
Who ever sheds the blood of man,
by man his blood will be shed;
for in His own image Genesis 9:5-6
But if a serious injury results, then you must require a life for a life—Exodus 32:24
Here is why God did not demand an eye for an eye at that time,
Where there is no law there is no transgression. The law had not yet been given about murder until after the flood.