Satan's Little Helper (2004)

During Halloween, theater student Jenna Whooly returns to Bell Island to spend the holiday with her family and her boyfriend Alex Martin, a classmate of hers who was also raised on the island. Her younger brother Douglas "Dougie" Whooly is very attached to Jenna and is fascinated with the video game "Satan's Little Helper," and he becomes jealous with the presence of Alex. He leaves home wearing his costume of Satan's little helper, seeking Satan himself on the streets so he can ask him to kill Alex. Dougie meets a serial killer who, coincidently, is wearing a costume of Satan and decides to help him. He brings the insane murderer home, and Jenna and her mother Merrill Whooly believe that the masked killer is Alex acting like Satan to please Dougie. Later, the boy walks with the maniac along the streets, helping him to terrorize the locals.