I know I probably will be called sexist scum for arguing for killing female character, but there is far more important things than politics.
"half decent morally casual gamer" might be the key word (word chain) here. I'm role playing a good, generally nice character, who kills, only, when it's really necessary (of course, I could kill lots of people there, but if I can talk them into/out of stuff, why lower yourself to unnecessary bloodshed), even if Gratiana did, what she did, my character doesn't judge people for their past, but for what they do in the present. Gratiana gave the Seekers roof over their heads, helps them, heals (well yes, that's questionable, I haven't seen her healing anyone, all healing was done mostly by me, but anyway, she's a priestess, healing should be her job as well) them, prays for them. No one is beyond forgiveness. And yes, the magic word "experience" the game is all about it, kill, kill, kill, kill everything that moves, collect EXP, level up and kill more stuff. Who cares about story, right?
The D:OS2 have a lot possibilities of role-playing and I sure that there was some players who just killed absolutely everyone in locations after finishing all the quests, just to maximize the experience.But if you, for example, role-playing good (albeit "good" and "evil" is relative concepts, see Hottentot morality) character and helped Gratiana it's would be the same thing if Batman saved the Joker, although Gratiana is so much worse : killed and tortured thousands of innocent people and shaped Braccus Rex in the monster he was and (spoiler warning) is today. I don't see how is that are redeemable. Even if you think that she changed and another person is today, try, in her quest, give anothers person soul jar instead of hers. She will belittle and insult you, because for her protagonist is just usefull idiot to do what she can't or don't have the guts to do. Gratiana appointed herself a priestess of Amadia, but goddess don't even talk to her, but do that for the player the very first time she/he goes to the sanctuary.
EDIT: and while Gratiana doesn't appear in the game later, she does appear in the ending, if you let her live. Your choice will affect events and you can see, that she really changed, same with Windego, if you let her go/save her, she'll redeem herself, she'll help elderly and sick people, or something like that (I saw that ending only once to be honest).
EDIT 2: (sorry, it's the last of me, I hope at least XD) you can call my character naive for god's sake, but that's also a role to play, she's nice to people, helps, where she can, earned a reputation of a hero, elven friend, dwarven friend and some more (yes, those are tags in the game, but why not call them reputation for the role play?). I'm not keen to earn a villain tag for lots of killing anytime soon, that's simply not my play style.
Everything she does are only for herself. Like people who donate money to "PETA" just to feel better about themself, and this organization use this money to actually kill most of the animals they allegedly "save". If there no animals left, people couldn't be cruel to them.
