For the definitive study of this issue, please see
Sorry; that’s not logical to me. I believe that God is One, just like the Bible says, Belief in the kind of dualism that juxtaposes God and a devil may as well be a kind of ‘demon’ itself, in mens minds.
Vermont Lt is correct in post 30 - humans don’t need supernatural help to screw up. Their unwholesome imaginations do that job just fine.