The only thing I like about Astarion is the voice actor's voice. I've tried several ways to kill him.
Don't do it in camp, those not in your current party attack you too
Once he's discovered to be a Vampire, about to turn into what I can only think of as a Vampiric Illithid is not a good thing. Especially since he seems to want to hang onto the Tadpole. But, I also mostly play a Cleric of Kelemvor, who wouldn't tolerate him to exist.