Why the ending was cheesy
I think many people are confused by the ending because the director uses one of the most cheesy and confusing literary devices ever concocted. The untrustworthy narrator. This movie reminded me of the last episode of Rosanne. For the later part of the series, they win the lottery and do all sorts of cool things but at the very end of the series, you discover that she was just writing a fictional story, and in fact a bunch of crappy things happened and they didn't win the lottery...really cool right? SO basically, you can write anything, show anything to the viewer and can totally negate everything that happened, but saying the narrator lied. This is not new, in fact writers have been doing it for years. It is a cheap trick, used to create outrageous and seemingly unrealistic events in the story for cheap thrills, then justify them at the end by saying it in fact never happened. In this movie however, the device is used just for a cheap twist, similar to the Uninvited (which was a lot better).
The problem being, when people read or listen to a narrator, that narrator becomes the expert on the situation on the events they are describing. It is a documented human condition. It is the same reason when we watch a documentary on WWII, we are assuming all information that is given to us is correct. Same goes for teachers at school, expert lecturers all the way down to the guy who works in finance, giving you stock advice at the dinner table. We automatically submit to their experience, authority or expertise. Most people do not recognize a first person narrator in a story or movie as untrustworthy unless you are familiar with the actually literary device. You automatically accept there experiences as fact, since they are there and you aren't. And that is why people get confused.
If you read what everybody is saying about this movie, and they things they don't understand or plot holes it all relates to the fact that they saw or were told something, and they took it as fact. Why was the killer driving while Marie was trying to rescue her friend? Where did that truck come from is Marie got a ride with her friend to her house? These questions don't prove they don't understand, it just proves that the literary device being used is ineffective.
This is why the ending is cheesy. These kind of twists are best left in third person narration, where things can be explained logically and be based on the events that have been shown to the reader/viewer. The ending then, can be backed up by the whole plot of the movie without confusion.
That is my 2 cents.