I think it's important to keep in mind that we actually know very little about Astarion's background. All of the stuff about him being a corrupt magistrate who sold people to Cazador was cut content/got re-written and nothing of it appears in the final game, which makes it non-canon. I don't think that backstory would serve his character well anyway, to be frank. Him becoming a horrible person because of 200 years of abuse and slavery is a lot more interesting than just being a monster from the start already. Him being a slaver who became a slave turns his story into one of punishment rather than tragedy.
All we know for sure is that he was definitely a magistrate. I assume he was probably already an arrogant asshole before he was turned, and maybe corrupt to some extent, but not a terrible person. We can see from his reaction to seeing Sebastian and his story about how he was punished for refusing to bring someone back to Cazador that he still had at least some morals and a conscience at that point.
What's interesting is that one of the hag's vicious mockery lines (or maybe it was a different context, I don't remember) about Astarion calls him "a ratcatcher who became a judge," heavily implying that he comes from a poor background and somehow either worked his way up to become a magistrate or simply deceived his way into the position (I'd bet on the latter). In the Idle Champions of the Forgotten Realms game, Astarion's character sheet has "Forged Patents of Nobility" under belongings, which supports this theory. But since that isn't officially part of Baldur's Gate 3, I wouldn't take it as canon, moreso an indication of what the writers had in mind.
In any case although I'd like to know more details about his past, I doubt we'll get them. A huge point of Astarion's story is starting a NEW life post-Cazador, and in his graveyard scene he makes it very clear that the old him is dead and gone. I don't think Larian has any intentions of dredging up his former life.
