The killer isn't real. He exists only in the mind of Marie. The "killer" is nothing more but a character apart of Marie's Dissociative Identity Disorder (a.k.a. Multiple Personality), which was formed from Marie's secret and violent obsession and affection for Alex. This secret crush for Alex gave Marie a violent side, making herself proclaim that she won't let anyone come between them, not even her family. So that's why Marie, the killer, slaughtered Alex's family, only putting an image in her mind that it was someone else who did it. In the last scene, we come to the image of Marie in the hospital room from the beginning, only for us to realize that it's really a mental institution. It then becomes aware that the entire movie was being told from Marie's point of view on how she saw everything, but she's totally unaware that she was the real killer all along due to how psychotic she is. Alex is shown standing outside the room looking at Marie through a two-way mirror. Even though Marie can't see Alex, she knows she's there, as she opens up her arms in a loving gesture.
Other mistake: When "the killer" is pumping gas, if you count the clicks associated with a liter going into the tank, they equal a total of 33 liters of gas, not 27 which is what the pump stops on.
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