I do not kill companions just because I disagree with them about ethics (or anything else, really). It's their life to live, not mine (man, how often did I use that sentence in RL debates lately). If they did something I can't tolerate, I'd still prefer to go separate ways rather than kill them.
Having said that, if things turn out, by the way I navigate the plot, that we end up on opposite sides, then that's too bad. I'd like to come to know Minthara, but since I won't side with the goblins as a rule she'll likely end up dead almost every time.
Everyone else, I hope to keep alive.
Even Astarion? He's a bloody vampire spawn ... which means he's evil and rotten to the core with no redeeming features whatsoever. Allowing such an abomination to exist is unthinkable.
Who speaks there? A particularly fundamentalist paladin? I judge everyone as an individual, and so far he's done nothing that justifies killing. Being a vampire doesn't mean anything but that you require blood to survive and you have certain unusual traits, such as not dying from old age. Have you played BG2? Those liches you destroy on your misplaced quest to get Kangaxx his bones? They were actually good, they had sacrificed themselves to guard those bones beyond their mortal lives in order to prevent the evil uber-lich Kangaxx from returning.
Undead abomination are abominations though. There are actually a loot of people that have a "kill on sight" policy for undead. Druids (Spore-Druids dislike intelligent Undead), Followers of Kelemvor, Lathander and a bunch of other deities.
And generally, good-aligned undead are extremely rare and need specific circumstances. Vampires on the other hand actually have drastic personality shifts upon becoming Vampires. Though I am unsure if this extends to spawns.
(Yes, I do kill him most of the times when he tries to drink my blood..)
