Oh he** no, self-fullfilling prophecy coupled with time travel, that's such a messy setting. He knows of the demon king in the future, then he goes back in time so there should be 2 demon kings, but if it was always only him as demon king then there was no demon king before, so he couldn't have known about the demon king in the future and wouldn't go back in time and all this wouldn't even kick off.
If you write about time travel you absolutely need to make sure that it follows the rules and makes sense, this is nothing an amateur webnovel writer should touch
