Cain would not bring a blood sacrifice to make atonement with God. Ironically, Cain required blood to appease himself.
Thus he elevated himself to the place of God, following in the footsteps of his father. But Cain’s sin was a step beyond his father’s sin; Adam prematurely seized the mantle of judgment (which God later gives to Noah in Genesis 9:6), but Cain exercised that mantle sinfully.
Perhaps Adam was even reaping from his own sin in this. Adam’s sin resulted in the death of a son (compare Genesis 22; Leviticus 1:5, literally “son of the herd;” 2 Samuel 12; Jesus).