Astarion is a 300 year old vampire spawn, slave to a tyrant basically, who got reduced from a position of power in Court, to a slave that pretends they have power somewhere. He was forced to feed on disgusting things and had no freedom. Then suddenly he gets power and a sense of freedom.
The guy's a walking time bomb, it shows, and Larian i think is purposely giving red flags about him, if anything, i think we haven't seen anything.
You have to understand however, which i'm sure you do, that he is an actually evil and narcissistic character, that was suppressed in what he can do through fear for 2+ centuries.
One thing i agree, is that it should probably be game over if the he kills your main char. Also, any normal real life character would chop his head off 5 times already. Even before we get to that. The character is well writtern in my opinion. The only problem stems from actually being able to revive your char after they die.
Actions should have consequences. If you allow a starved and tortured vampire for centuries to suck on your blood, while he was not allowed to have human blood all this time, then it's not gonna end well. Vampires are notoriously bad at controlling their urges.
Still, you shouldn't be able to just "rez lol", and if you can, the others should cause a scene.