The Dark Hours
The ending to this film is open to interpretation, but what is certain is that Harlan (Aidan Devine) and Adrian (Dov Tiefenbach) have not really invaded the cabin. This scenario is an invention of Samantha Goodman's (Kate Greenhouse) brain tumour-riddled mind as she slips into a coma induced by a deliberate overdose of the drug she had tested on Harlan while at the asylum.
What is unclear however, is whether she really did catch her husband and sister "at it" at the cabin and killed them herself, or whether this is also part of the delusion. As we never find out who is banging on the bathroom door, we'll never know for sure.
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Read more about this film at The Internet Movie Database.