Astarion is so evil More I explore his dialogues the less hope I have for saving his soul. The Myrina ending clinched it for me -- he thought the returning her husband in the form of a zombie was hilarious.
Pretty basic emotional reaction -- he finds the pain of others humorous. Maaybe there is a redemption path but it sure looks like he's planning to become a stronger and eviler version of his master.
Yeah, I think you are right all in all, but the nuance in my mind it that he does not seem like actively evil or actively seeking to cause harm. Still, you are right that his lack of empathy = evil alignment. He is the kind of evil person you can deal with, get on your side, and make useful. Like a criminal that can be cool with his friends and allies.