If you do not do well, sin is crouching at the door

Genesis 4:1-26
The Lord said to Cain, "Why are you angry, and why has your face fallen? If you do not do well, sin is crouching at the door. Its desire is for you, but you must rule over it."
Cain killed his brother Abel. God punished Cain, "Now you are cursed from the ground, which has opened its mouth to receive your brother's blood, it shall no longer yield to you its strength. You shall be a fugitive and a wanderer on the earth."
Seth also a son was born, and he called his name Enosh. At that time people began to call upon the name of the Lord.
Adam and Eve, after their expulsion from the fatten, have two sons:Cain, the farmer, and Abel, the shepherd. In due course each brings his offering to God. Abel's is accepted, but not Cain's. It was not what Abel offered, but his faith which made his gift acceptable. Cain's better  resentment shows a very different spirit.
Cain kills Abel - it is a short step from rebellion to bloodshed - and God condemns him to a nomadic life, but provides protection against death.
The last two verses give a glimmer of hope. Seth is born to Adam and Eve, and people begin 'to call on the name of the Lord.'
Why?
As the prophets will say again and again. God is not bought off by sacrifices. He wants the offerer also to 'do well'. True faith cannot be divorced from right behaviour.
How?
There is hope from Seth, and God wants man do well more than soffer sacrifices.