My own personal theory on this is Belkar chooses his death. Maybe it's because in some major battle to come, the IFCC calls V to keep them out of the fight, and Belkar, knowing the wizard will be more needed than he is, decides to go down there and rescue his frenemy the only way he knows how: with violence! Of course, he has to die to get down to the Hells, so he tells old Durkon to look after Mr. Scruffy and makes sure he dies somehow. He fights his way through the lower planes, rescues V, and decides not to come back, on account of the fact that he's having the time of his afterlife killing every demon and devil he meets. I think that would be a nice ending for him, after a fashion.