I would say stalking gone, well, worse. After all, it's not at all impossible that a stalker would eventually kill the person they were following once they stopped living up to expectations. In this case though, I guess it'd be more like killing her because your dere side lost.
Edward supposedly stalks Bella because other supernatural scary critters can kill her easily, so he wants to protect her. I think it would be more Twilightesque if the vampire lost control of his appetite and eats her, as I heard that the possibility of this sort of situation causes Edward some angst, but also causes some Fridge Logic. If he's so worried that he'll eat her or vamp her or whatever then why does he follow her around?
Vampire can't control himself? Possibly the relationship could cause a fight between the best friend and main character?
In one of my stories, the protagonist is designed to be a vampire slayer, but doesn't know it yet. Her best friend met and fell madly in love with a strange guy, who will turn out to be a vampire and will end up killing her.
Given that I've decided to model the best friend and the vampire after Bella and Edward, what would be a good way for their budding romance to lead to the best friend's death?
If I'm asking for advice on a story idea, don't tell me it can't be done.