He seems to divide the players community into two fronts, those who hate him and those who love him. No matter which side you're on, he evokes strong emotions - and that just shows how well written he is. Especially since people could just avoid him, but why would anyone, since he makes us feel so much?
Additionally he's quite interesting to observe as well:
I am not sure, if his background story has changed, but he was intended to be a terrible person from the beginning. He was a corrupt magistrate, who then, ironically, ended up as a slave and spawn. Now I would assume that someone who suffered would learn something from it and develop empathy, but that's not necessarily true. Moreover it seems, Astarion has turned into a sociopath, because he was made one and henceforth doesn't feel anything (except for some minor thrills like praises, sex and power). Also he wasn't an ordinary slave. He was stripped of his free will, paired with abuse he was probably forced to enjoy. For having undergone this, he seems weirdly stable to me, but maybe, by being sassy and fabulous, he copes with his traumatic experiences. I mean he was captured in the freeze state for 200 years, and hells, maybe acting like a douch is the only thing keeping him together. Maybe he just dissociates and tries to avoid facing his past with his master, and thinks, by killing him, he will be whole again, free of the taint of being controlled. Imo it will get interesting, once he killed the vampire, because he can't channel his hate and guilt towards his tormentor anymore.
I'm super curious, how his story will continue and what's gonna happen to him, I just don't want him to break my heart, I'm not ready for another Solas, I am still not over that.
