I don't think Astarion is pure evil, but he's still evil. As Wormerine said, he's sadistic, cruel, petty, etc. But I do think that considering he was, until we met him, undergoing centuries of torture, humiliation and enslavement, he deserves a little compassion. He had literally no agency for 200 years or so. He went from Cazador to the nautiloid and Wwhen we meet him, he can probably count the HOURS he's been free on one hand. Because of that, I don't think we can really make a judgement on him being pure evil or not. That sort of thing would twist anybody into the worst version of themselves. It can't be overstated that he physically could not resist Cazador's instructions, so its not even a case of "he could have chosen to disobey." He literally was not in control of his actions, and
At the same time, I don't think calling him harmless is accurate either. Firstly we have the gameplay issue of Larian probably not wanting the companions to act entirely outside our control as the players. So I think that on a meta level, some of his "good behaviour" is down to that. Also there is the fact that, again, he has not been able to act with his own agency for literal centuries and was not freed until literally when we meet him. So part of it is likely also that he's just...spineless, for lack of a better word. He does what we say because he doesn't want to get on our bad side and doesn't want to burn bridges with potentially valuable allies. He did very clearly try and feed on us without our knowing, so when he does take his own initiative, he's hardly faultless.
Ultimately, I think Astarion is redeemable and deserves some grace, but any redemption he might gain would require a combination of genuine compassion yes, but also someone firm enough to not indulge his worse aspects or let him go crazy with his freedom. Someone who can empathise with how much of his attitude and outlook is probably a trauma response, but who will still hold him accountable for what he does now that he's free and able to choose for himself.
Having sympathy for someone like Astarion is quite subjective. I get why you and some others may feel that way, but I have zero sympathy for him. Ditto for SH. In fact, I find it very difficult to relate to any of these characters and find all them to be utterly detestable and unlikeable.
But making evil characters not really evil after all (it's what passes for character "depth" these days) and having them be "redeemed" is the biggest RPG cliche and trope out there nowadays. It's what's in fashion, so I guess it is to be expected that that is how each and every one of Larian's characters have been written.
