I almost take offense to this comparison, because the similarities end with 'stabby elves with evil former masters'.
I say this as someone who genuinely likes Astarion and has him in my party constantly.
Even at her darkest, Sebille never scratches the surface of how genuinely just....morally bankrupt Astarion is. Every person Sebille kills was in some way involved in what happened to her, being captured, sold, stripped of her dignity- sure maybe the role was minor, but even the most minor role involved searing a mind control scar onto her face to make her a living weapon. When Sebille lashes out at you, she generally recants her behavior- she understands she's traumatized, shattered by the life she has been forced to live. While she's cold, calculating, she is hardly ever cruel, she possesses empathy and compassion that Astarion, from what we've seen of him currently, lacks.
Astarion is willing to kill anyone, for any reason, killing is fun for him- he relishes in rampant slaughter, is manipulative towards you from the moment you meet him, and generally tries to play you like a fiddle for his own ends. I have no doubt Cazador is an evil bastard, and that Astarion's main desire is offing him so he can be free- but unlike Sebille, he doesn't really have a structured plan, nor does he particularly care how many innocents get caught in the crossfire. He is rather apathetic towards slavery(talking to him in the miconid colony yields some interesting dialogue), despite being essentially a slave himself- and his actions range from petty, strange, to downright hypocritical. He seems like more of a homicidal teenager let loose than a former slave on a bloody quest for revenge.
I know we haven't seen much of Astarion and we've had an entire game to get to know Sebille, but if I were to compare Astarion to anyone from DOS2, it would be The Red Prince.
I, initially, didn't care for The Red Prince, he was snobby, self interested, refused to believe any of my good actions were for the actual sake of good, and despite me agreeing to help him take down a shadowy master orchestrating his downfall, took a while to warm up to me and be upfront with his whole story. Sound familiar?
Red's story sees him grow from a pompous, pessimistic blood monger into an introspective, enlightened ruler who cites his friendship with the PC being one of the greatest values in his life. Sure, he's still rather out of touch with...the normal world, but he grows a great deal and stands by your side staunchly while godly power is waved in his face.
Like I said we don't know nearly as much about Astarion and co. as we do about Red and Sebille, but I feel Astarion's plot formula mirrors Red's more closely. Both coming from places of looking down upon others and seeing themselves as 'greater' despite essentially being in the same mud pool as us, as well as a general apathetic view towards the lives and struggles of others.
I agree with this post wholeheartedly.