User blog comment:IamJakuhoRaikoben/Chapter 356: Tartarus Arc, Review./@comment-5343732-20131019064909/@comment-7418318-20131019221507

Maybe the comment about Silver "devouring" demon souls is the giveaway here. He might kill fellow demons to absorb their powers or somesuch.

Or there really might be a difference between Demons and Devils. Remember that his powers were translated as Devil Slayer Magic, so he might be a demon who slays devils.