Well to start off, I think that when playing a D&D game it is very reasonable to expect a D&D vampire. As for not following the standard alignment chart, I'd say that he definitely does, and he is definitely evil. If you help the tieflings, he'll outright say that being a hero feels awful. That's cartoonishly evil.
I'm not objecting to the scene being well written, I just think that the part where he kills us and 'apologizes' afterwards is basically slapping us the player in the face and at that point it doesn't make sense for us to keep him in the party. If that's what Larian's intention is, to provide a point of no return where players will just find sticking with him untennable, then they did a great job.
Another issue with him killing us that isn't related directly to Astarion though, is the fact that it really messes with the canonicity of death in the game. In most crpgs, deaths are just non-canon. But here, you can die and be brought back and there's a specific reaction to it, cementing death as canon. And I worry that it will have weird knock-on effects down the line. There's already the issue of why our tadpole doesn't leave us when we die.
