Satan's Little Helper (2004)

On the afternoon of Halloween, Douglas Whooly (Alexander Brickel) dons his costume in expectation of a fun-filled night. But, when he hears that his beloved sister, Jenna (Katheryn Winnick), has found herself a boyfriend, he throws a tantrum and flees the house. Douglas befriends a man in a devil costume and helps him decorate his lawn with fake corpses. Unfortunately for Douglas and his family, the corpses -- much like the murders that will punctuate the evening to come -- are all too real.