I would never compare Astarion to a Prince. Their common feature is only the love of power (and not same way). Otherwise, they are very different. And I understand why the OP sees common ground between Astarion and Sebille.

1. Master
2. Scar
3. Desire for revenge
4. Strong distrust

But Sebille is not a manipulator. Astarion manipulates, after the first attack, he immediately becomes a cutie. He says "trust me", and can kill you in the same night. He's also a chaotic evil, and I really hope that doesn't change. I can expect that even if MC supports him, Astarion will still betray us. But it looks more pleasant than "the way of redemption".

In DoS2 end Sebille became softer, and I rly don’t like it. I liked who she was, but not who she became. I hope that Astarion will follow path of evil, even if it ends badly, so be it.

I also didn't read spoilers, I don't want to spoil my fun.

I don't speak english well, but I try my best. Ty