Imo Astarion is just less hiding about his nature, meanwhile e. g. Gale will compliment your character on their good deeds if you play a hero, so a lot of players consider him good. But after taking Gale on an evil playthrough at best I could say about him is that he is a neutral character who abandons his principles after one conversation.
The only companion whom I couldn't keep on the full evil playthrough was Wyll*. So it looks like to me we have one companion who is good-leaning, and the rest is either evil, or neutral with weak morals.
*Not counting bugs here. I've had one conversation with Zevlor where he asked Wyll something but Wyll's response didn't trigger, and the tieflings in the cave went hostile. But this dialogue worked on patch 4, so perhaps there are other instances where companions don't react properly yet.
