But this games' "Sebille" is way less enticing and way less forgivable.

With exception to 1 companion in DOS2; they were all neutrals to a degree, with Lohse edging on good.

With BG3 - if Astarion is this games version of Sebille (Even an Elf to boot, go figure?) there's not even a smidge of good. Plus he's undead. Big no-no's in DnD universe. If he has a redemption option, it's certainly packed in the late game.