Every day, we - each of us, are living out the story of our life. You know why human beings don't kill people? Well, obviously for a host of different reasons, mostly ranging from the Judeo-Christian belief that to take a life is immoral and wrong to "Well, I'd like to kill that person, but I simply don't fancy going to gaol for the rest of my natural life." I would imagine that vampires, if they actually existed, would be faced with the same set of issues with killing that the rest of us have, especially considering that in practically all cases of vampires portrayed in works of fiction, they were once human beings, and so the fact that they had to come to terms with the harsh reality that they need to feed to live must have been difficult for them.
Imagine if the world went to pot tomorrow and civilisation were to crumble... How many people do you know that can take down a deer with bow and arrow? How many could successfully field dress a fresh kill? How many would know how to butcher it properly, to preserve the meat, to see that practically no portion of the carcass goes to waste? I'm guessing not very many. Would you be able to adapt yourself to having to hunt for food? Would you be able to wake up every morning knowing if there isn't a rabbit or a squirrel in your trap or a deer in your sights you'll go to bed with an empty belly? What if there were no animals to hunt and no sustenance to forage for other than your fellow man? Would you be able to knock the door at your neighbour's and drag someone out into the garden and take his life so that you may live?
Even if you didn't have any other choice, I'm sure that there would be people at the bottom of your list - perhaps the girl or boy whose name you scratched into your pencil box at school... Perhaps the doctor who brought you into the world... Perhaps the nice old lady who always gave you strawberries from her garden... And likewise there would be people at the top of your list. If I were going to kill someone - for ANY reason, I'd prefer to kill someone that at least, in my eyes anyway, actually somewhat deserves it. I'd kill the person who knocked my kid brother about or an old boss I never got on with, or someone in a Man U shirt, that sort of person.
Why wouldn't vampires have the same reservations and predispositions when it comes to killing? They'd be just as likely as we mere mortals are to want to kill people who may have wronged them, or people who might be seen to be less desirable to have around, and they might be inclined to spare those who have shown them some degree of kindness, or people whose presence they feel makes the world a better place for whatever reason. If I were a vampire, I'd kill people who lack an imagination, because who needs them or will miss them anyway?