God saw that Abel had been obedient and had brought the right sacrifice - the firstborn of the flock - and looked on the obedience favourably. He also saw the disobedience of Cain and showed displeasure at it.
Cain did not want something or someone to pay for his sin. He was a good person and his willingness to give God a present that had cost him dearly should have been enough. His pride stood in the way of reconciling with God. In Genesis 4:6 we see God's response to Cain's anger.
Once again, admitting he was originally wrong went too much against his pride and he refused. The result was as God said. Sin was at his door and it took him prisoner. So he went from being a sinner, alienated from God to being a prison of Satan and God's enemy. This has serious consequences.