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 ^^
