He can't be the devil, he can be devil like, remember, the devil never had all the powers of God, he was just a powerful Archangel like Michael, but not Godlike.
Just cuz Cas has 30 million souls and can waste archangels with a snap of his fingers doesn't mean he's God. He just thinks he is. which in a way is a devil. Another similarity with Lucifer. Lucifer blew up Cas the same way Cas blew up Raphael.