Genesis 4

 After the Fall of Adam and Eve, their first child, their son Cain will commit murder over his younger brother Abel due to jealousy. Abel pleased the Lord while Cain displeased Him. Abel obeyed God and did things according to the way required by the Lord. Cain did not honor the Lord but did things according to his own convenience, not according to God's instructions (V 1-8).

The Lord confronted Cain after his crime. God pronounced a curse over him but He mercifully protected Cain's life too (V 9-15). Note that all Cain's descendants will later be wiped out during the Great Flood in Noah's time (Genesis 7). Noah descended from Seth, a new son born to Adam and Eve (V 25).

Later, Cain's descendant Lamech will commit bigamy and would also become a killer of man (V 16-24).

Adam and Eve would bear another son, Seth. He would in turn bear a son named Enosh. Then this generation would begin to seek God finally after all the rebellion and sinning of their forefathers 'generations. (V 25-26).