The Pursuit of Cain
By faith Abel offered God a better sacrifice than Cain did, by faith he was commended as righteous when God gave approval to his gifts. (Hebrews 11:4)
Abel's offering was an act of worship, giving God the best of his flock. However, Cain's offering did not involve faith and was seemingly just a religious exercise.
In reality, Cain is actually angry with God because his offering was rejected. Overcome by anger, Cain lashes out and kills Abel despite God's prior warning that "Sin is crouching at the door, but he must master it!"