I'm in agreeance with N7Greenfire about the notion of Minthara being redeemable to be ridiculous. Furthermore, you don't just massacre people, you help kill the children too. Anyone who thinks that can be spun in a positive light is concerning. If you want her as a companion, that's your prerogative. Just don't pretend such acts are justified by trying to redeem a single person, or that said person can be redeemed in the first place.
Also, unless it's been changed, iirc Astarion was a corrupt magistrate that secretly sold people into slavery (or something similar), namely the Gur, who rightly got their revenge by jumping him and nearly killing him. It was then thqt Cazador "rescued" Astarion. As to where the information came from, it was either data mined or the friendly Gur tells us (leaning towards the former). The information is really old at this point, so who knows!
