I finally get this movie *spoilers!*

I really liked this film, I love twist endings and identity crisis issues. After seeing this film, and are still confused, read on.

Alice was traumatized as a little girl and she invented multiple personalities to cope with the stress. All of the "girls" in the ward were fictional and in her mind.

The "ghost" was Alice's real personality killing off the other personalities and trying to take control (or you could say it was the doctor's new treatment pushing Alice's real mind to come out). The scene where Alice was killed by the other girls represented them suppressing the true Alice personality, thus creating the Alice ghost in response because Alice was never truly "dead".

And notice all of the other girls were killed except Kristen, who was the strongest of all the personalities imo. The battle between Kristen and Alice ghost where Alice is "killed" with an axe indicates that Kristen is a personality here to stay and won't be "cured" so easily as the others. Thus, Kristen comes back in the end to fight a battle with Alice, who is not cured.

Offsreen, this tells me that Kristen will eventually regain control fully while the real Alice will slowly be suppressed. Brilliant.