The demon slayer is a demon himself. It actually makes a bunch of sense: It's not like demons would have any loyalty or love for their own kind (they would love nothing at all but themselves), yet they might know their own kind the best, which would make them expert slayers of demons, provided their power is sufficient, which shouldn't be a problem for this guy. On the other hand, these demons were made by Zeref, so only Zeref knows what sort of characters they are. They seem to worship Zeref as well, despite the man himself not having done overly much for a long time, I believe.