Did this a couple of weeks ago and had the same problem.
This is one of the bugged quests, I killed the guy and he despawned before I could loot him. Sent a ticket to the GMs, but they couldn't help me as this is an old and known issue, and they have it to solve on their backlog.
I came back later, alone, and it happened again.
The way I finally finished that quest was to ask a couple of friends to do it with me, and I didn't touch him until he died, when I looted his corpse.
Try it that way.