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.
I disagree. An abuser who became a victim and his worldview crushed (probably mostly thanks to Tav) is much more interesting to me. How to treat such person? Give him the second chance of punish him? It's fascinating and ambiguous. The abuse cycle is common trope and more black-and-white, a victim who was an abuser - not so much. I'd love to see this version of Astarion's story - it was the reason why I bought this game 3 years ago. Joke's on me I guess.
