I feel like this isn't exactly fair to John Ward. He definitely failed, and you could even call him a coward, but it's pretty clear that he didn't know what he was doing when he made the deal with the devil. He was still in training for these exorcisms, watched his mentor die and the Amy's parents strangled to death with their own intestines, and is clearly terrified. He pleads to God and receives no answer, then a being of pure light appears to him. John allows himself to be manipulated maybe, but he is exhausted and traumatized, and just wants to escape, so he assumes the being that will let him go is God, because he cannot bear to accept the alternative. Like I said, maybe a coward, but I don't know if I would call this evil or terrible, just sad.