To be fair, if there was a species known to actively kill or eat humans 99.9% of the time, we would certainly try to clean them off the face of the Earth. Just because this special singular specimen exists doesn't suddenly invalidate the tendency for the rest of that species to kill us.
That said, since they're now dead set on becoming "Good (TM)", I'm just gonna assume that these were originally created by the omyoji to gain power. Or that they were originally buddy-buddy with omyojis and then got backstabbed by some omyoji ages ago. And, because they said that the parents weren't eaten but cremated, they're probably alive or were a part of an internal struggle to become friends with the monsters.