[Hunters - Slayers]
Firstly let me begin by saying that killing any human being, especially because you think it's somehow 'your job' or you don't like their lifestyle etc is murder, and not justifiable under any circumstances. I really don't care what you call yourself. We are all subject to the laws of the land and a jury isn't going to aquit you or pat your back for a job well done in saving the universe because your defence is 'well, s/he was a vampire!' Though what you might get is a nice safe padded room.
Do I believe they exist? What I am sure of is that out there are many deluded/psychotic/religious freaks who have overdosed on Buffy...and who will gladly tell you of their great struggle with the 'evil undead' and how they are 'coming for you'. Now, while in general these are laughable creatures throwing out their threats over a computer there is a more sinister contingent who may not stop there and may target any person they percieve fits the 'vampire' bill, attacking or harrassing them in reality. These are obviously the more dangerous and it's advisable (even if it's just repeated online harrassment or they make it obvious that they know where you are) to document or record all conversations, keep logs of IM's and descriptions of any actual physical confrontations so should it come down to restraining orders, police etc you are well equipped to state your case. Do remember that threats etc online are taken just as seriously as those made face to face, but for the most these can be nipped in the bud by reporting the offender to their ISP, blocking them from messangers and mail - simply not responding and taking away any need for them to feel they must.
**never, ever put yourself in a potentionally dangerous situation by giving out any personal information such as actual address or phone number with a 'come and get me then' bravado because it may fail you should they call your bluff!
I don't believe we as vampiric people are supernatural, 'otherworldly' or inherently evil so quite frankly these people are looking for what doesn't exist.