20/20 hindsight (spoiler as if you don't know already!)
I watched this again today with the knowledge of what happens at the end, and it still didn't help me accept what we see. I watched it trying to keep in mind, a) the killer is imaginary, and b) it's all Marie.
I found it difficult. I was still gripped by the pursuit of Marie by 'the killer' for instance. It did make sense of why Marie says 'I felt like I was chasing myself', the mother asking Marie 'Why?' as she dies, and why Marie cannot remove Alex' gag, and why Alex freaks out whenever she sees Marie.
But in terms of the whole thing making complete sense if you know there is no male killer, and it's really all Marie, I'm not convinced.
Still a good film though.