Cain produced evil works because he yielded to the spirit of Satan rather than the Spirit of God. His deeds were evil while his brother's deeds were righteous. The works produced in a person's life serve as evidence of which spirit that person yields to and is influenced by. Cain brought forth evil fruits and evil deeds as a result of this influence. God rejected Cain and his offering because Cain produced works that were evil. Those who do not practice righteousness produce evil works and are not of God.