All right, at this point I just want to say we'll have to wait and see if Damon really does snap and kill Caroline. Given his behavior and hers in recent episodes, that doesn't seem likely, especially if he's trying to win Elena's trust back. Katherine, of course, could still kill Caroline without a thought, because she's the one who's been using Caroline and who Caroline is now trying not to obey. I don't expect that to happen soon though.

And why on earth should that be a fact? Why should the writers allow that?

The short answer is - because this is a show about vampires. If it was a show about humans or emotionally balanced villains, we could contest it more and ask for more. Damon has already proven he acts rashly when emotional, and I'm sure he's not alone in that (look at Katherine's actions, Anna's after her mother's death, Stefan's when he's on human blood). It's not a justification - but it is the explanation. If the writers ignored the fact that vampires are supposed to be selfish, emotional and dangerous I would checking if this was Twilight.

I want to be very clear - I am not condoning a vampire killing someone because they don't like them - being a vampire does not remove a character's culpability. It does put it in a different context.