And now it seems I can't complete 3 sub quest. I have no idea how he died, since from what I remember I killed the spider Queen first, then met Frederick. And lo and behold after a while I return and find his corpse. Is this is a bug?
I'm pretty sure convincing the imp to talk to Frederick has something to do with his demise. He never seems to die before this happens; he always seems to die shortly after.
There is a good chance he died in spider Queen fight. I have done it several times, and he usually enters the fight, and some times he get killed by small summoned spiders far, far away.