Its really no wonder why he hates her and this kingdom/world he's gotten nothing but bad experiences from them. Treated like something less then human despite being one, clear death threats one after another, no reward stated after doing what he's been asked to do. Him turning fiend isn't exactly a strange choice considering his circumstances honestly it sounds like the most fun option.
*sips tea while watching* Im just here to watch the Chaos unfold - my pfp sauce is Luno from the game Wuthering waves