I think a simple re- edit could make this WAY better.... {spoilers}
Cut out the scene where the police spot Marie on the security footage and move it to the end.
If the killings are told from Marie's point of view, why would she tell that part? How would she KNOW that part?
Let the car chase continue. Marie supposedly defeats the serial killer (her personality re-asserts itself) and she returns to the van to free Alex. Instead of seeming grateful and relieved; Alex attacks Marie and runs. Marie grabs the buzz saw and gives chase. (Don't switch back to the old man. ) At this point the audience may be confused. Is Alex just traumatized or is something more at work here? As Marie jumps on the car killing the bystander the audience begins to realize something more IS at going on. Marie professes her profound and unstable love for Alex. Alex eventually stops Marie ...with a crowbar through the chest.... and then we see Marie is in the hospital with the staff and police showing her the surveillance video. The last shot is that little glance Marie gives to the camera.
viola'.....
what do you think?