The ending could have been absolutely perfect if... [spoilers inside]

... they revealed the twist slightly later, and ditched the old man once we know the truth.

Let me explain:

We understand she's been the one killing others when we see the security tapes.

Now, what would have been great is if we realised the truth when she's trying to free her girlfriend Alexia. We would have been stunned if we saw Alexia threatening Marie with a knife after been freed ("WTF are you doing, she just saved you!"). And at this moment, we'd see the cops watching the tape. Bam.

Also, I think that once we know the truth, it would have been way more creepy if we never saw the old man again, but instead Marie shifting personalities.

What's the point of the old man once we know he doesn't exist?

Kinda ruined the ending for me...