So the princess had some kind of twisted desire and love towards the hero. But I think that's normal, everyone thinks of that kind of stuff once in a while. The difference is between thinking about it and actually doing it.
For example you might be walking around the street, see someone and think about punching them in the face for not reason whatsoever. Or thinking about grabbing a baby and throwing it as hard as possible. I know this is a normal thing but I forgot the name of it....
Either way, what the Saintess did was pushing the princess to go through that barrier between "thought" and "action". She amplified her desires to become even more twisted. Was the princess already fucked in the head before this? Yeah, but now I can't help but to think that out of everyone else, she was the least bad person, unless the Saintess was also controlling the rest of characters but I don't think that's the case.
EDIT: I found the definition of what I was talking about, "The Imp of the Perverse"