Having sympathy for someone like Astarion is quite subjective. I get why you and some others may feel that way, but I have zero sympathy for him. Ditto for SH. In fact, I find it very difficult to relate to any of these characters and find all them to be utterly detestable and unlikeable.
But making evil characters not really evil after all (it's what passes for character "depth" these days) and having them be "redeemed" is the biggest RPG cliche and trope out there nowadays. It's what's in fashion, so I guess it is to be expected that that is how each and every one of Larian's characters have been written.
I think Shadowheart is a actually different ballgame to Astarion. Sure there's hints that she's a brainwashed Selunite, but it's only that, hints. So based on what we know of her, she's a willing Sharran cleric and has participated in all the misdeeds that one would expect. So from that perspective I don't think there's any reason to hold back judgement for her. Although as an aside to that, the game really doesn't explain what Shar is and just how evil her worship is. As someone unfamiliar with the lore, I didn't really understand how truly evil Sharrans were until I got to the underdark and saw the description of that dark feast thing, where they would kill a selunite or good god worshipper each day for however long. So I think that with Shadowheart the game's biggest flaw is that it fails to communicate that Shar is genuinely evil and not just 'dark and morally ambiguous, but still posessing a valid role in the cosmic order'. As for Astarion, he's absolutely an asshole and if left to his own devices I have no doubt he'd run amok as an ammoral menace, killing and ruining and doing whatever it takes to amass power, but I also think that based on what we know about his character, there's nothing about him that suggests he's irredeemable. If you take into account the fact that he literally was powerless to resist Cazador's thrall, then as far as we know he's actually not done anything evil before we meet him. Not by choice. He likely would have just gone on as a normal magistrate had he not been turned. That being said, if you don't like him, you don't like him, and you don't need to justify why. I don't particularly like him either, but I'm also a bleeding heart who loves hopeful stories and believes everyone deserves at least one chance at redemption, even in real life. Though I think a fun twist on his apparent character trajectory would be if he was just an asshole even before he turned. Maybe he was actually a corrupt magistrate, he was beaten by people harmed by his corrupt rulings, and Cazador's treatment only amplified his prior terrible traits. I think that would be quite a fun reveal.
Regarding what's in fashion though, I feel like the fashion nowadays is sort of the opposite, making it that seemingly good characters are actually morally compromised and having the lesson be that 'nobody can truly stay clean in the real world, and the grown-up way to look at the world is to accept that everyone is kinda bad.' I think real life has been giving people a cavalcade of evidence that things suck and nothing will get better, so stories saying 'hey things don't have to suck and people can change if they do the work' is a refreshing message in the face of that.
