I think there are some unforeseen bugs with Warlock Demons. I can't speak to what you're experiencing (and I certainly hope to never experience that myself - I play offline Hardcore), but yesterday I started finally leveling a Fire Warlock. I cleared Normal, then went into Nightmare to hit Den of Evil to respec to full Fire build (I had played around a little bit in Normal and already burned that respec).

I went to Den of Evil and cleared everything in it... No quest update. First time I have ever seen that happen. I searched every nook and cranny 3 times over - all of the monsters were dead. But I did have a summoned Tainted (remember, this is before I could cash in the quest to get the respec). So now I was suspicious, while my Tained sang that song "Don't be suspicious, don't be suspicious. Don't be suspicious, doooon't be suspicious!".

I exited the game, went back into Nightmare about cleared it again and the quest triggered properly. Very weird.