I guess he needs a little bit more training before he can dream of being called the demon king. The problem with giving him such OP skills is that he doesn't actually suffer much. Only suffering could really refine his hatred and remove his humanity. But lets be honest, it didn't seem like before this moment he suffered anything other than hunger and lack of rest. I trust the goddess isn't planning to let him leave too easily. Otherwise he might still side with humans and others opposing the demons. The MC should be pushed far more than this and then pushed some more. One big problem is that he hasn't needed to kill any humans yet. The goddess should create a good, and very forceful, opportunity for that.