While I do find Astarion's (cut?) backstory intriguing, and I wish they'd worked with the idea that was introduced and seemingly forgotten that his death was a deliberate setup (the PC can point out to Astarion that Cazador just happening to show up seems super coincidental, and in the same conversation Astarion says that Cazador works through proxies and usually has several layers of plans going on)...
...I think making him that outright evil from the start damages the cycles of abuse theme his story currently centers around. The
whole Cazador-Velioth scenario, and Astarion's ascension,
very deliberately illustrates the process of victims becoming their abusers, when in this case Astarion would start out as an abuser and I suspect that's why it didn't make it in the final game. To clarify - Astarion was always a bad person and we can infer from several things in the release game that he was a terrible magistrate. But there's a difference between 'spoiled noble who didn't care who his decisions hurt as long as he got to enjoy prestige and power' and 'selling prisoners to vampires for the promise of becoming a vampire himself someday.'
So I'm... curious but hesitant on this. I really wanted to explore more of Astarion's past as a magistrate, and I wish Cazador had been shown as more of the powerful lateral schemer he was suggested to be. But I'm much fonder of a spin like 'Cazador bought out Astarion's career and bribed him into doing increasingly reprehensible things, and Astarion went along with it because his life was great and why should he care about other people's problems' than I am in wholesale resurrecting 'Astarion wanted to be a vampire from the start and everything that happened to him afterwards was pretty much karma.'
Granted, it'd be a fun layer on how much he hates the human servants in Cazador's household that sought him out willingly (because Astarion is a huge hypocrite and that'd be the best case yet), but I just think it breaks the core theme of his story if he was already exactly that bad before Cazador got his hands on him.