... you obviously killed Astaripn since he attacked you ...
Why is that obvious? Well, if you equate being evil to mindlessly killing everyone in sight, it is. But I think good or neutral characters can viably kill in self-defence too. Or even as part of an eye for an eye attitude.
Truly interesting evil characters, that is to say, smart evil characters, will keep others alive so they can be betrayed, blackmailed, used as pawns, etc.
In the case of Astarion, his vampirism is weakness that a smart evil character should want to exploit for her/his own nefarious purposes. Hopefully the game will accomodate such lines of thinking.
