J.T. Petty has a canny awareness of the underground horror genre—too canny, in fact, to convincingly sell his documentary about the subculture as a work of pure vérité. Thwarted in his attempts to profile a local peeping tom, Petty instead opts to investigate the world of super-low-budget horror movies, where fantasies of rape, mutilation, and humiliation reign, and where the illusion of raw authenticity differentiates its hardcore material from Hollywood’s glossier scary fare. A cinema professor and two psychologists discuss issues of voyeurism, compulsion, and deviance to intriguing if familiar ends in S&Man (pronounced “Sandman”), while “Jesus Christ: Serial Rapist” exploitation mastermind Bill Zebub (get it?) admits, “I shoot it so perverts give me money.” Examining the motivations of those who make and watch such extreme entertainment, Petty aims to implicate his own audience through myriad gory clips that exude a simultaneously repulsive and absorbing pull. It’s a mildly clever agenda, albeit one that crumbles when Petty speaks too directly to the genre’s “Is it real or fake” appeal via a subplot concerning Eric Rost, the shady creator of a killer-stalker series (“S&Man”). Implausibly posited as a potential murderer, Rost takes the proceedings off the rails, and Petty’s film devolves into a transparent and uninformative gimmick.