I don't see him as pure evil because of compassion and understanding. I played with Astarion in my party multiple times and he doesn't do a single bad thing in my playthroughs nor do I have any issue getting his attitude high primarily making good decisions. Sure he has his opinions on certain matters, but that's pretty much all he does. He's pretty much harmless as long as he's with me and he's behaving himself.
If he was pure evil, then he would gut Gandrel on the spot without a moment of hesitation. He'd feed on the innocents and he'd try biting us even after saying no. But he's being a good boy instead, despite his issues. Maybe if people gave him a chance and actually took him along for a playthrough, they'd see he ain't so bad as they thought ^^
He has to kill to live. Heck, he can even kill Tav but what does death really mean in a world where you carry around ressurection scrolls
He doesn't outright attack Gandrel because he would probably stand no chances with him, weakened from living off animals and the whole tadpole business. I don't think he would think twice if he felt a bit stronger.
We have no way of knowing his true intentions but everything he does seems like playing it nice and keeping with the group for as long as they protect him. Nobody from the companions we know right now would willingly stick around if they didn't personally need it.
