Deus Ex Machina
The ending was really upsetting for me as it exemplifies Deus Ex Machina.
By suddenly saying, "It was all a dream...", one is able to resolve any plot holes by saying they were just in the imagination of the antagonist (Marie).
The problem with this is it is allows for shoddy writing. The twist ending could have worked if the story had been structured differently. There was definitely a promising beginning to the film and many aspects were well executed (pardon the pun...).
They should have written it a little better. This would have allowed the twist to work without needing to redact large parts of the story as just being imagined.