Genesis 4:15

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 Samuel 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. 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.)