I know this is old and you all probably long since cared about this, but I just wanted to make something clear. The Onis WERE most likely a threat in the past, especially if we factor in the fact that humans didn't always have those powers. Keep in mind even the beginning of the story state that the Momotara grew strong enough to scare the Oni into hiding. So naturally what are seeing NOW are watered down kindhearted love having Onis. Things aren't so black n white in stories like these, Onis could have easily been depicted as horrible if we saw them in the past before humans could fight back.
Though blonde hair guy deserves to be torn to shreds still cause on either side that just messed up.