I see most reviewers got this as insanity or overall disturbia. It's not my particular reading, but It's definitely not impossible.
We're talking about a teenager with perfect mom and dad who's room, clothes and stuff is very girly decorated, but seeing past the graphic violence this movie has a lot of sexual tension - all you have to pay attention is the the naked bodies, the licking tongues, all the bloodletting and rubbing... I don't want to sound like a perv and I'm not talking about sex action, I'm talking about sex identity.
Being a teen sucks badly by itself and most families only make the experience even worse. The fact that you're ready to start a sex life (and you usually want to do so) doesn't make it a lot easier also.
To me this girl was in her room all the time, dressed - just as her mother found her when she was taking the tea to her room. The wholeaction was inside her head: The violence against still being a child, the hate for herself and her dilemmas, the way you don't know how to react, the desire to escape at any cost: Who has never felt them?
In my mind I've stabbed a LOT of people. Myself included.
My point is, this girl is not some demented psycho - she's just normal girl, but she's seeing the darkest recesses of her mind. Check your own "chest of secrets" for a while and tell me if what you find there is a lot better than this.
Mine is worse.