[Just mythoughts]

Cain is an interesting subject. Note he is not listed in Genesis 5:1 THE BOOK OF THE GENERATIONS OF ADAM.

Cain has his own generations with sound alike (but spelled differently) names.

Also Cain's punishment was Genesis 4:11 And now art thou cursed from the earth, which hath opened her mouth to receive they brother's blood from thy hand;

12 When thou tillest the ground, it shall not henceforth yield unto thee her strength; a fugitive and a vagabond shalt thou be in the earth."

Yet Cain received protection, v.15 "Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold." And the LORD set a mark upon Cain, lest any finding him should kill him.

[Elsie]

It appears that God is giving a pre-cursor(sp?) to "Thou shall not kill"!