Cain fell out with Abel. He seems to have blamed Abel for his own problems with God. There’s a close connection there. If we aren’t dealing with our sin properly, we will wind up sinning against others. If we wrong God, we will wrong others. Anyone who claims to love God, but hates his brother is a liar.
Working through problems with others gives us a clue to honest repentance before God. Cain was supposed to keep his brother. Not just keep track of where he was, but protect and help him. Instead he killed him. He needed humility to admit his wrong, but got angry and took it out on others. He needed to resist the urge to envy and hate and condemn. We need write off our sins to the cross account, so we can receive and keep our brothers. If we cling to our sins, we will wind up writing off our fellow brothers and sisters.
Let us confess our sins before the Lord.