Left for Dead (2007)

The students of a Canadian town throw wild parties full of drugs, sex, and lost inhibitions every year at Halloween. But one Halloween, an uninvited guest comes for a visit. He wears a Halloween mask while he decimates the pleasure-seekers with a machete. Tommy, a party-goer, witnesses the murder of his friend Freddy, but neither the police nor friends believe him, and the corpse has vanished. Tommy harbors a terrible suspicion as the very bloody spectacle continues.