פּירוש אויף בראשית 4:15

לכן כל הרג קין VERILY, WHOSOEVER SLAYETH CAIN — This is one of the verses where the construction is elliptical (literally, which abbreviate their statements), which give only a suggestion and do not fully explain. “Verily, whosoever slayeth Cain” express a threat, suggesting the consequence — “thus shall be done to him”, “thus and thus shall be his punishment”, without explicitly explaining what the punishment would be.

לכן כל הורג קין, “therefore, because Kayin has already been so humbled,”

לכן כל הורג קין שבעתים יוקם, “therefore whosoever slays Kayin, vengeance will be taken on him sevenfold.” The plain meaning of this verse is that seeing that Kayin was worried about being murdered, G–d assured him that anyone murdering Kayin even after he had lived seven generations would be held accountable by Him. This is why Lemech who held himself as guilty of killing Kayin, even though he did so unintentionally, told his wives what would happen to anyone killing him on that account (Compare verse 25 in this chapter.) Lemech’s children were the seventh generation, after Adam. (Adam-Kayin-Chanoch-Irod-M’chuyael-M’tushael-Lemech-(Yaval,Yuval Tuval Kayin) He told his wives that the whole generation of such a killer fathered by himself would be held responsible for his murder by G–d, seeing that what he had done was due to his being blind and therefore totally innocent of any intent to harm a human being.