I think they have potential to change their ways. It's not a new thing, even in bg2 companions could change alignments. Vioconia was really bad initially. This is a character I remember when I got her thinking what the hell but you could keep her even if you were good and eventually she becomes neutral. So I don't see why shadowheart can't. Spoilers from here - so far I learned that she loves Shar but one significant detail (that is even written in the journal entry) is the fact that she gave up some of her memories. My guess is that those memories will factor in her "alignment shift". I don't see what's wrong with it if it is well written.
