Her dad and deception, ground rules, police procedure
Spoilers...
My problem with the movie is the dad and the (presumably) good dead cop with the hole in his head. Neither of them helped her. The way I see it, if evil can use memories of loved ones to trick you into avenging their death, then her (real) dad's soul/ghost/whatever should have been able to communicate with her and at least try to help her. Warn her.
The other problem with this film is the ground rules were not laid out. It wasn't always clear what was real and what wasn't. That's not a problem in itself, but taken too far, it cheapens the ending. For example, the "evil" should be able to conjure up illusions, fine. But to mask out something real like the clean up crew's look and sound is just too easy. Too convenient. I guessed that ending anyway, but it still felt cheap. If evil is this powerful, then it can literally make you do anything it wants. Hell, it could've made her call in the entire police force with some bogus report and have her murder them at will within 5 minutes of the old cop leaving.
Also, when she was shot without warning, the old cop didn't even warn her to drop her weapon (unless it's the omnipotent evil tricking her again). She had already shot the cleaning guy in the head, so it's not like old cop had to shoot her to stop an imminent threat. He just arrived on the scene and did not know if she truly acted in self-defense or was neutralizing a terrorist plot, etc.
Started out promising, but the repetition and story went downhill. Nice acting by the lead actress at least.