You've all completely missed the plot
I have read the forums and people's take on making sense of this film.
I'd like to point out that I am French and I have seen the original version.
Everything is said in the first 5 minutes of the movie and you've all missed it.
The film opens with Marie at the mental institution about to tell her story of what happened. And she proceeds to explain she was chased in the woods.
She then wakes up in Marie's car saying she has had an awful dream about being chased in the woods. Adding: *the weirdest thing was that it was ME chasing ME*... clearly stating that Marie is NOT insane.
The next, and 2nd time the bad guy appears is when ALEX wakes up and says: *I swear I saw someone in the corn field*
The third time the bad guy reappears is when Marie is about to go to bed....
Every time the bad guy appears, is when one of the protagonist is near or has just slept. It is done in a very subtile way unlike some American movies of the 80's (which were the inspiration for this film as stated by the director)and this is in fact where the twist is.
The idea of Marie being in fact the killer, is a very modern idea and does not explain anything. (hence all the plot holes if you want to go down that road)...
The twist is in fact the most simple explanation: it was all just a bad dream.