Interesting idea, BAD execution (spoilers)

I thought the plot - or at least, what I could puzzle out of it - was actually kind of interesting; however, the whole thing got bogged down with way too much randomness.

Synopsis: The land where the house sits was 'cursed,' either by one of the tortured slaves or the torturing itself, to cause insanity and death in anyone who happened to go there. From the doctor's comments (know what you're looking for, etc) it's possible that having a 'purpose' or not believing in the curse protects you from it. Twenty years prior, a disturbed teenager (possibly under the influence of the curse, though they never mention whether he knew about it or not) strangled the family dog, then stabbed his father with hedge shears and torched his mother alive. The boy's body is never found.

Present day, five college kids - Kristy, Sharon, Eric, brothers Matt and Tyler - 'sneak' onto the property so Matt can film it for his thesis. Kristy and Sharon are girlfriends; Kristy seems to have a jealous streak when Sharon acts friendly/flirty with other girls. Eric and Tyler are apparently studying film, and have lots of specialized audio/video equipment. The brothers seem to have had a bad childhood, with references to Matt being beaten by their father, and a possible family history of insanity. They arrive at the house and split up, with the girls going off in the woods while the three boys go to set up their equipment. After some arguing, Tyler goes downstairs for photographs and takes an apparently disturbing polaroid before hearing what sounds like an argument/fight between his brother and Eric. Going back up, he finds Eric missing and accuses Matt of hurting him; Matt insists he did nothing, and they go upstairs to investigate some odd thumping noises. In the attic, they discover Eric's body tied to some wooden stakes and mutilated, swinging back and forth and causing the thumps. Tyler suspects Matt of "going crazy again," they argue for a few minutes before Tyler turns and gets caught in a large steel trap. Matt tries to draw out the killer they believe is watching them, but Tyler's body disappears, and he starts to believe Tyler might be pulling some kind of stunt. With the attic door stuck, he goes out on the roof and jumps down, apparently breaking his leg.

Meanwhile Kristy and Sharon have been walking; they come across a dozen or so steel traps left open in the trees, and free a rabbit that gets caught in one. Kristy cuts her hand on the trap, then can't find Sharon and begins calling for her. Sharon comes up behind Kristy, both girls are startled, and Sharon gets her foot caught in another trap. She insists that she WAS answering Kristy, even though Kristy apparently didn't hear. They head back to the house, making it just in time to see Matt drag himself to the front door, covered in blood from Eric's body and leg broken. He tries to warn them away from the house, but gets dragged inside by a figure in black, while the girls fall through a hole into some kind of underground passage.

Sharon - having alternating flashes of bitchiness and terror - believes the figure in black was Tyler, while Kristy insists she saw someone in a dog-faced mask. They break into a tunnel connected to the passage and end up in the basement of the house, where they barricade the doors and wait for the figure to come down. They also find Tyler's photography equipment, including the disturbing polaroid, which shows a misty figure in a dog-faced mask. Sharon degenerates further, her attitude alternating between bitter anger and crazed fear; she argues with Kristy, pointing out her "need to be right" and hinting at Kristy's attraction to Matt and/or Tyler, without considering her (Sharon's) feelings. She also begs Kristy not to let her die, while Kristy reassures her it won't happen. The figure breaks in, and the girls overwhelm him, only to discover most of the doors to the house locked and the car dead. With the figure in pursuit, they hide in the woods away from the road, hoping to trick him. Unfortunately, he catches up with them as they head toward the sounds of a highway, and Sharon is stabbed to death with shears. The figure attempts to drag Kristy toward the 'slave shack' seen earlier, but she escapes and runs across a field to a local farmer's barn. The farmer calls the police for her when she asks, but gets killed by the figure right after. The figure then chases Kristy across the highway, where he is hit by a semi and (apparently) killed.

The final scene shows a new graduate student speaking with the same professor Matt spoke to at the beginning of the film; this student is majoring in criminology, and mentions the incident where "Kristy Goodman went crazy and slaughtered all her friends." According to her, the masked-figure's body was never found, and the driver of the semi didn't remember hitting anyone. The professor insists that Kristy never murdered anyone, that she's simply traumatized.

General Plot (my take): There are a few possibilities. One, the curse caused Kristy to go insane, possibly when she cut herself on the trap; she murdered all the others and hallucinated the dog-faced figure / the earlier boy. This explains why Sharon didn't see the figure either time she should have, instead seeing Tyler.

Option two, the curse made the boy from twenty years before go crazy. He murdered his parents, then either lived on the land or was 'consumed' by the curse, becoming a phantom or ghost-like entity. It then took the forms of / entered the others, and murdered them. In this case, Kristy getting cut might have acted as protection against the curse, which was why she saw the dog-faced figure instead of Tyler; she saw the creature's true form. It would also explain how Tyler's body "disappeared," if the dog-faced creature could manipulate what its hosts/victims saw.