AFAIK, you need -6 morality for Carys to start complaining and whooping -12 for her to leave. I have little idea even how to get it that low, let alone why. Like half the quests are morality farms - this fact allows you to up your morality so many times you have to be really purposeful to gain a big minus.
I mean, I'm in ActIII, I used guide to get almost every possible morality debuff and I still have, like, +8 or something, just because I do not refuse quests, many of which almost certainly ups your morality. The only way I can imagine is to act like a douche in almost every dialogue and refuse many quests altogether, but that would render your playthrough short of many rewards, which is just impractical.
Anyway, since I hit almost all morality debuffs, corrupted all my party members and generally acted like a cruel greedy asshole, I consider my playthrough a corrupted one. Yet I still have positive morality and do not worry about Carys leaving.
So, you can go corrupted and have her in party since it is quite damn hard to gain needed minus morality for her to leave.
