The answer is right there ... you quoted it:
... you obviously killed Astaripn since he attacked you ...

I just dont like it ... there is nothing deeper in it.

If you want to play character who takes with him someone who litteraly put a knife on his, or hers throat ... then be my guest ... but every single one of my characters WILL kill Astarion right at their first meeting, bcs he threatened their life and that is unforgiveable ... especialy in situation where the agressor is apologizing in the way Astarion does, bcs he dont seem to mean it, more like it seems he is mocking us, and that is second time in mere minutes where he crossed the line.
Well, maybe except Paladin, or Monk those will knock him out.

Well, if you equate being evil to mindlessly killing everyone in sight, it is.
Oh not at all ...
Simple elimination of threats.

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.
Yup.
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.
Depends ... truly interesting and start evil characters in my opinion should allways calculate in order to find a way to use someone to their benefit ... yes ... but they should also calculate if that person is worth the risk ... someone who dares to attack you is not, at least not for me.
In the case of Astarion, his vampirism is weakness that a smart evil character should want to eploit for her/his own nefarious purposes.
Maybe ...
But in your first meeting you dont *know* about his Vampirism ... actualy if you ignore the dead Boar, you never find out.

And why exactly should "smart evil character" examine any dead animal you meet?

Hopefully the game will accomodate such lines of thinking.
Agreed ...
I know people love redemption stories for Evil companions ... i personaly love corruption stories for Good companions ... hope we get at least one. :3