Cain, the eldest of Adam and Eve’s son, tilled the soil while the younger son, Abel tended the sheep. Each makes an offering to God. God “respected” Abel’s offering but not that of Cain.
Here is another case in which God is setting in motion what follows: jealousy, rivalry, and bitterness.
The murder that follows, Cain’s banishment, and the bloodline follows seems to be necessary. Hence “genesis”; origins. It’s also worth noting that as God curse’s Cain with a fate more than he can bare, he turns around and protects him.
And the Lord said, Therefore whosoever slayeth Cain, vengeance shall be take on him sevenfold. And the Lord set a mark upon Cain, lest any finding him should kill him.Genesis 4:15
Our hapless man once again is an instrument of creation as God populates the earth and establishes the building blocks of society and the human condition.