I'm not sure if you can really say most, that's a negative stereotype of a murder hobo party, which does happen, but that's not all D&D parties and this isn't a story about a murder hobo party, so it's hardly relevant.
As for if the vampire would be worse then Belkar
The vampire spirit is your worst day, personified. However bad Belkar is, some of his days are better than others. Can you imagine what his worst day was like?
For a goodly person like Durkon his worst day is the day he was wronged by others and everything he knew or desired was turned to ash, creating a resentment and seed of evil within him that he buried deep down. For Belkar, who appears to be evil for the evilulz, that might be very different. Belkar at his worst might actually be significantly less evil then normal Belkar because unlike a good person like Durkon, evil doesn't represent what's the worst part of him, at least to himself! And his worst most traumatizing days might be when he wasn't very evil at all. Belkars worst days may very well be the days he spent under the curse, weak, sick, reliant on others and hallucinating. This lead to almost the opposite reaction that it did in Durkon, leading to a deep down seed of... If not good, at least realization that evil isn't really the best path forward for him. A vampire nurtured on those memories and feelings could come out very differently then a stereotypical good person turns into evil vampire.