I think Astarion has some potential as a character (he has clear values, agency, and goals as a character), but as it stands, his overall antagonistic actions/views towards the "standard" player and the party really makes it hard to like him as a companion. He really should just be an NPC (likely enemy)
As of right now, I feel like he's kept in my party mainly due to meta reasons (i.e. his face is one of the characters on the cover of the game, hence important, don't kill).
In general, the string of interactions you have with Astarion (in particular the his dumb plan and hostile introduction and plus the boundary breaking, creepy attempt to drink your blood) really forces a player to ask the question as to - why am I keeping this person around? Regardless of your player's alignment.
The blood drinking interaction is particularly heinous for any alignment of player character because he's actively attempting to harm a party member, an ally in battle. Even "evil" parties need certain limits and boundaries to co-exist properly. Not only is his actions boundary breaking and hostile (again), the fact that he was dumb enough to choose the main character as a target for this "first" attempt (who is a useful ally and can actually fight back) also shows a certain level of incompetence that is unattractive as an asset. E.g. the fact that he chose you to attack, over any relatively harmless prey (i.e. random, frail commoner X) or maybe even just staying behind to "finish off" one of the numerous wounded enemies the party faces, shows an inability to control his impulses and a lack of tact and strategic decision making. Even for evil main characters, Astarion is very likely a liability at best, and an antagonistic force at worst.
He is also the only companions that don't offer you something or a reason to keep him after being the only initially hostile companion. No cure solutions (Lae'zel). No knowledge (Gale). No artefacts (Shadowheart). No history being a comrade-at-arms (Wyll/Shadowheart/Lae'zel). The only reason why he joins the party is because he suffers from the same condition, but his eventual goal even shifts towards wanting to keep the parasite, which basically misaligns him with the main quest and casts of the game.
I think the easiest way to illustrate how "off" Astarion feels is to compare him to another more "blatantly evil" character - Lae'zel. Despite probably being willing to kill us with no hesitation, it makes way more sense for us to work with because:
1) She wants to be cured too 2) She offers a solution to cure the parasite 3) Your relationship with her starts off fighting side-by-side (in most cases)
It's very easy for a good or evil character to rationalize keeping Lae'zel in the party.
It really boils down to this. I really have no reason to keep Astarion around, and the story seems to actively push him towards being an antagonistic force, regardless of my character's own alignment.
I think the evil path in BG3 has issues in general, and in some ways, Astarion (being the most evil of companions) seems to suffer from that same issue. Maybe Larian is aware of this and that is why we're suddenly getting more interactions to exile/betray/kill Astarion.
