Astarion is one of the evil companions. He can mellow down a bit, but he will never become a good guy. Same as Minthara. If you don't like that, you have a lot of possibilities to get rid of him.
Yeah to me it has always seemed like this is the way anyone playing the game even remotely in a good way should handle Astarion. Just cut off his head. Or better yet, recruit him first then execute him for his crimes, which I personally believe is what is appropriate for most of the so-called companions in this game. It is unfortunate that I can't do this, though, to all those companions.
