בראשית ד:טו הלכה: Contemporary Halakhic Problems, Vol III

And the Lord said unto him, "Therefore whosoever slayeth Cain, vengence shall be taken on him sevenfold" (Genesis 4:15). R. Nehemiah said, "The punishment of Cain is not as the punishment of other murderers. Cain killed but he had none from whom to learn [the enormity of his crime]. [But] henceforth whosoever slayeth Cain shall be slain."