Fair points, but you know her end game. These wraiths, you do not know what they have planned. They devoured enough souls and that just
made them good? 
More than likely they are far more insidious than they let on, because again,
they gained their power and sentience by eating souls. Does that sound... familiar? Like how every kitsune has their
soul devoured? And their "god" lies and says that it's just "keeping them safe." They aren't good. They're more or less treating the people of Savarrah like cattle. They gained sentience and realized "Ah jeez, if we let some
live they'll go on to make more people. More people = more souls to eat = more power and food for us."
Sure Kas is a demon and does demonic things, but is the devil you know not better than the devil you don't?
Edit: There's even that bad end that explicitly showcases the devil you know is better, as you don't know what Tollus would do if he bested Kassyra without his bad end. It turns out, he's way worse.