@Sandy
I'm 75% sure it was one of the demons that killed the poor wolfie guy, and 99% sure that irregardless of who killed him it's a demon that left his corpse out like that.
the humans are there to negotiate for a more lasting peace (accepted as being not very likely, but technically still one of their goals) and to at least try and figure out who the hell the other demon guy was working for when they were manipulating the dragonling. in both cases they gain absolutely nothing by killing a demon AND leaving the corpse on the demon lord's castle's front doorstep
setting aside the entirely legit possibility he sought out a fight and got killed in return as the cause of death, it's much more likely that either a demon with a strong grudge against humans or possessing excessive pride and viewing it as worthless to keep peace set his corpse there to incite an incident, or possibly an agent of the very secret sect of demons MC and Co are seeking out did it to delay the would be heroes, whichever way, there's far more likely reasons for the staging from the locals for the corpse's positioning