The Satan Man is the villain from the movie Satan's Little Helper.
A sadistic and perverted arsonist turned serial killer, who tried to burn down the police station of the town in which the movie is set, only to be arrested and later get bailed out of prison by his dad, the mayor of the town. After being set free at the time of Halloween he put on a black robe and grey devil mask to hide his face and stopped talking to avoid people recognizing his voice (he never speaks in the film, and his face is only seen at the end).
Now called "The Satan Man", he at first went around from house to house killing people, and later convinced the movie's main character, the young and naive Dougie Whooly (who was obsessed with the very poorly made video game Satan's Little Helper) that he was Satan himself and that he was playing a game by pretending to kill people. Dougie wanted to go along and be like the game character, helping "Satan" kill and not knowing it was real. Satan Man later kills Dougie's dad and he learns that the killing was real (though he still thinks he really is Satan and not a normal killer).