I agree with you, OP. Not only is it squicky, the encounter is a bit nonsensical. If you accept but don't pass the persuasion checks he ends up killing your character, so if your character didn't wake up to find him creeping in the first place, was his original plan just to feed on one of his allies, at the risk of killing, right in the middle of camp? Though, it's not like anyone jumped to the defense when you spotted him anyway... sleepin on the job :P
Something else I recall when you catch him is that he says something about having to do it this way because he needed your trust? Which doesn't make any sense. I also think it's a good idea if the scene were changed to be the character seeing Astarion sneaking out of camp again (which the camera shows for the boar encounter) and they find him snacking on animals.
