I'm gonna go with Ernest Hemingway here. Really enjoyed reading his books that in my opinion are excellent pieces of literature, but at the same time the man himself was a self-centered, alcoholic wife-beater, and egotistical prick that back-stabbed his closest of friends.
It sucks too when you find things out like this, because you get so invested into their works and end up imagining that they're a great person and that their material reflects their personality, only to find it as the complete opposite. It makes it hard to continue appreciating their art, but at the same time I don't like letting a person's personal life affect their professional work, but it's kinda hard. Still, despite the crap he's done I do respect his writing.