Reminds me a lot of the French movie High Tension

http://www.imdb.com/title/tt0338095/?ref_=fn_al_tt_1

In that movie, we follow a protagonist who is visiting her friend's house out in the country, only to be attacked by a brutish killer.

BUT...

It's revealed at the end that the main protagonist was actually the killer the entire time, and that the hulking male killer and the entire plotline was a fantasy the main protagonist had created as she did the killings herself. She also had some violent personality disorder that made her oblivious to her own actions until the finale.