Cain's offering was accepted....just not emphasized and a big deal made over it by God. Abel's was.
Cain was accepted for his offering....as well as his offering.
Abel's was just miles ahead of Cain's. That's why God was making a fuss about it.
And as the Firstborn he was supposed to set the standard. But his runt of a brother had showed Cain up big time.
Interesting, Cain killed his brother over jealousy, Abel being a better person than himself, Being accepted by God, feeling not accepted by God. When God accepted us all, no one better than anyone else, ever since day one in the fall of the first Adam. God has loved us all even in our failures, otherwise Son would not have come to earth to reconcile us all first in his one time willing death once for everyone (Hebrews 10:10) or would he have?
God warned Cain that sin (Unbelief) to God's love for us all, is real. Cain, distracted by not doing correctly, another person (his brother) Accepted and Cains not accepted took him to anger over the thoughts entering in his mindset, wanting to be accepted and thus to be a better person that even his own brother, is Satan's goal for us the people to be independent on God's plan for us his creation, that he continues to love in mercy and truth.
Think on that one Luke 18:9-14, I see God hates pride, one, anyone, making themselves or even receiving thought(s) they are better than others is not good for my Soul at least, and is for each person to decide between God and them as well.
God told Cain, to be aware, Sin (Independence of self over the creator God himself is a mess)
We, all are given freedom to choose and Cain chose, having this freedom to choose, yet did it without against God's lead, and so it Was Anarchy, in killing brother.
Now we have belief to God, and still have the freedom to choose. And if one does not still have freedom in beleif then it is tyranny.
Cain did wrong, period, so have I, so has everyone else. God through Son as risen gives new life to be led by God freely to do right over harming anyone else ever again, at least this what I am now seeing, in my long walk off a short pier daily as dead to the first born me. Romans 6