KMC Forums - can't get this movie out of my mind

what is the scary movie where a young, blonde, short-haired woman, possibly in her 20s is invited to her friend's house with her family. A killer comes and she hides in the closet and witnesses the mother get her throat slit by an old fashioned razor and later finds that the dad got his head stuck in the stairwell and the killer killed him by pushing a dresser into his head thus smushing it off. The young son is killed when he runs into the corn field and the killer shoots him with a shotgun I think. I think the main character (the blonde girl) is actually the killer and I think she chases her friend into a greenhouse and calls her name out a few times (Alex I think). The blonde girl kisses her friend and the friend kills her with a crowbar by stabbing it through her stomach. I think it was made in the early 2000s but I'm not sure. please help me I want to know the name of this movie.

***also the blonde girl touches herself in the bed where shes supposed to stay