I think maybe people's idea of what you need to do to be evil is out of step with what it means to be evil. Being evil isn't a record of your deeds, it's your worldview, if you've never hurt a fly before in your life, but you think Might Makes Right, then you're evil. Just because Astarion or Shadowheart haven't twirled their moustaches while murdering children doesn't mean they haven't espoused evil ideology.
Being evil isn't a crime in FR, and having an evil worldview isn't incompatible with existing in a good or neutral society. In a highly pluralistic and polytheistic setting you have to expect that there's a live and let live dynamic that allows people of wildly different beliefs coexist, Baldur's Gate itself has a pretty prominent temple to an evil deity. We might mostly run into the wild eyed cultists, but there must be a slew of stevedores and clerks who go home after a long day at work, and say a little prayer at the altar of Bane before supper.
Astarion might have a pretty warped view of the world, but that doesn't mean he can't do good, or become a good person. I think it'll be interesting to see how the game treats how evil characters as they face moral conundrums where these worldviews are challenged.
Shadowheart doesn't fit evil by any standard. Hell we see first hand that she doesn't even do the the rituals required of Sharan clerics she probably isn't even one. Also swen was very carefull to not go to the selune claric at the harper camp and remembered last minute to not talk ot the shar doctor guy when he had shadowheart in his party. The while exchange there with the Narrative lead telling swen it was fine to do the interaction on his playthrough where his shadowheart bought fully into shar was telling.
At the end of the day there's a reason Wotc has moved away from the strict alignment system, but Astarion really doesn't fit chaotic evil either. He's closest to chaotic neutral and has demonstrated repeatedly he has a good heart when it comes to the party.
