If you use the speak with dead amulet on the dead body in the room with the traitor, he also says he's a warlock.
Also, there's apparently a passive Religion check during one of the dialogue trees that I failed....I suspect it has to do with their implication of a devil working with gnolls. (Gnolls are demonic so in lore, would hate devils). But I didn't pass the check, so I'm not sure.
I will say, I was a bit suspicious when they started implying the devil they were hunting was working with the gnolls. Speak with dead just confirmed it.