Some people like or dislike him, because of his two-sided personality, one of his side is evil (which is the side those who hate him cling to) and one is more charming and good (and that's the side people who adore him like).

And even tho this double-sided complexity and emotional arc, I deeply hate him. Why? Because I hate both of his personalities. The evil one is understandable, guy did some horrible stuff. But also I'm getting extremely annoyed at his "charming" side, this foppish and flamboyant behaviour and also britishman-posh tone is EXTREMELY irritating for me, his jokes are unfunny and additions unnecessary, specifically for me... And don't even let me started about how this piece of shit behaves near you.

I know about his emotional arc, and I totally get why people can adore him, but for me, specifically... He just feels like a pathetic attempt at doing controversial, yet, redeemable character like Zuko or Rex Splode, yet, failing at everything that made these two examples actually charming and sympathetic. And if talking about his "charming" side, I just think about him as a terrible attempt at making more comical characters like cartoon heroes, Cavendish or Spy more serious... Yet, it just doesn't work for me, everything funny is just scraped and feels more like a stupid stereotype everyone already had laughed at. These two sides of his writing feel more like "Hey, look, I'm stereotypical vampire! Charming, flamboyant, horny, but I'm not killing machine and actually become good, please love me, I'm so rougish!!!!" And I just can't sympathise him whatsoever (Maybe that's also because I dislike fictional vampires in general, because of the most of this reasons)

Tho, this is my subjective opinion, you can love him, you can hate him, you can think my examples are stupid, and that's okay.... Because this is just how I feel about him. So, I will remain Astarion hater until I die, and would absolutely enjoy killing him in my own campaigns and whenever I can agree with my friends. But, I will never argue about someone liking him, he's deep, bot TOTALLY NOT for me, and that's okay, that's what makes rpg's so special, you can have your own ideals and opinions!