That is a good point on how evil operates.
I would say I do have sympathy for the supposed "good vampire". The idea of a vampire who is a reluctant one and good in nature is, for me, an interesting and somewhat beautiful one.
I would allow PC's to become vampires however not lightly. Once you go down that road you take PC's into a total other world. As a GM I would rather avoid that whole aspect as it can get pretty confusing, which takes away from the fun.
I would say no I would not allow PC's to become vampires or such magical beings like this. Unless the party has reached a level that it would be relatively easy to fit in. Instead of being a super hero type character. For example once a wizard gets to a certain level it is mutch more powerful than a fighter. However if you give that fighter vampirism, it equals it out a bit more.
Granted as wizards become higher level they get to the point that they think they are unstoppable. There is nothing quite so fun as a GM to see a PCs face as they realize their 18th level wizard just died.
I think a person wanting to be a vampire has a lot to learn about the actual creature. It is a undead creature that feeds on the blood of humans or demihumans. Its nature is to kill. While some will thing a vampire can be good they are mistaken. A vampire is a evil creature that must be destroyed or the world as we know it will end.
Here I'd like to highlight a point I was meandering about with, earlier, and show you something which I'm reproducing from a paranormal site where I moderate, as an example of how "modern vampires" ("real ones") are viewed, by some:
So here is my definition of a vampire, although not intended to be exhaustive, or exclusive.
It is up to the GM what the powers an such are but after reading all this I think it is also up to the GM to explain his understanding of Vampires before introducing them, as many people have many different understandings of them.
I would also say if you have a vampire or vampires in your campaign you really have to rethink the whole structure of the political system and such. Vampires don't just sit in their coffins and wait for PC's to kill them. They try and control and acquire power just like anyone.
Once you introduce a vampire into the campaign you are introducing a lot of other factors.
Edited: Oliron on 8th May, 2010 - 2:15am
For sure if we know we're facing a vampire we need to like ask the GM what he knows about it but then again that might spoil the story cause really you're not supposed to automatically know what a monster will do because some things are just urban legends.