I'm in the camp that says "some reason" is "because he's Belkar". Nothing he does now is going to make up for everything he's done already. No last minute sacrifice is going to convince anyone that he's not still a threat if let off his leash. Nothing he does is going to make Roy say "You know what, I feel like having this albatross hanging around my neck for the rest of my life, let's pay to raise a known murderer."
Also, I agree. We know how vampires work now. Vampire Belkar wouldn't be Belkar, so either way he's dead and gone.