My guess is that somehow miku is gonna fuse humans and demons in a way that is not possible to reverse, that way killing the demons would have to also kill humans, that way the idaten will just let humans and demons survive as one specie;
At this point i am almost at the side of the demons, yes they kill humans left and right, but the idaten are even more inhuman than any demon.
The other option that would be a fun curveball would be a demon has a kid with a idaten, that way making a hybrid that does not need to be exterminated, that would be fun