Quote Originally Posted by lololink View Post
I think it's because the Oni (鬼) from japan culture aren't really demons, not the same demons we have in christian myths anyway, they can mean a lot of thing, either demon (understand evil creatures) or protector, they usually are fierce and really powerful warriors. According to the dictionnary the translation is fierce god so I guess fierce is the part that is retained through that kanji and not so the evil part.
In fact they are (were ?) the same. Christian demons (or rather, demons in short) were originally beings of a higher status than humans. Angels, fallen angels, dragons, witches, gods... All were demons. The name evolved in Christianity to describe the Fallen Angels first, then all malicious beings in the end.