Doesn't deserve the low rating it has on IMDB. I wonder if that's because of the ending.
I'll admit I didn't quite get it at first, and only had my eyes opened by a couple of informative reviews afterwards.
When she kills the couple, she isn't actually possessed by the demon. She kills them in order to entice the demon back to her. In her earlier moments of loneliness, the priest rejects her by not staying for her performance, Rose rejects her by not looking after her as the headmaster had asked, and so the devil takes control of her when she's at her most vulnerable (The devil inside her responds to Rose later on with "You had your chance!") Only this demon has shown her any kind of companionship, but breaks down at the end when she realises that even her crimes have failed to bring him back to her