What mark did YHWH leave on Cain?
Gen 4:15 - “Not so!” replied the LORD. “If anyone slays Cain, then Cain will be avenged sevenfold.” And the LORD placed a mark on Cain, so that no one who found him would kill him.
Note the comments of Ellicott on Gen 4:15 -
The Lord set a mark upon Cain.—This rendering suggests an utterly false idea. Cain was not branded nor marked in any way. What the Hebrew says is, “And Jehovah set,” that is, appointed, “unto Cain a sign, that no one finding him should slay him.” In a similar manner God appointed the rainbow as a sign unto Noah that mankind should never again be destroyed by a flood. Probably the sign here was also some natural phenomenon, the regular recurrence of which would assure Cain of his security, and so pacify his excited feelings.
The Cambridge commentary is similar:
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. Ezekiel 9:4; Ezekiel 9:6), 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 (Ezekiel 9:4; Ezekiel 9:6)—which was once doubtless worn quite openly by His devotees, and only afterwards degenerated into a superstition.” (Gordon, Early Traditions of Genesis, p. 211.)
The mark that God gave to Cain is not something like a badge. It means God endowed Cain with a special ability to be sensitive to threat of life. Samson had that kind of sensitivity and every time when Delilah bound her and whispered that the enemy were upon him, automatically his body reacted and he escaped. It proves that God is benevolent even to the worst sinner so that the sinner would repent and turn to God and be eternally saved.