J.T. Petty loves horror films, and he’s made a couple of them himself. His third feature, “S&Man,” is a documentary about the creepy world of underground filmmaking, and he’s done something fiendish. You’ll enjoy it more if I don’t tell you what, exactly, but think about this as you watch it: Are some elements of this documentary too bizarre to be true? Or are they so bizarre they must be true?

Normal people like you and me don’t know much about the sub-genre of filmmaking that is Petty’s focus, these direct-to-video movies that are shot cheaply and feature gruesome scenes of torture and murder. Many of them are meant to look like snuff films — i.e., like the person being killed on the screen is ACTUALLY being killed. You’d never suspect that was true of something you saw at the multiplex, of course. But something made by people you’ve never heard of, that you bought on VHS at a horror convention? Now it starts to seem plausible that the camera has captured actual murders.

The sick fantasy that the death is real is part of the attraction for some fans, and filmmakers like Fred Vogel (“August Underground”) and Bill Zebub (“Kill the Scream Queen”) are interviewed at length about their methods. Vogel talks about making fetish films for highly specific demographics, like men who enjoy seeing violence done to a woman’s navel, or who have a big thing for shoes. Contact Vogel with your fetish and pony up some bucks, and maybe he’ll make one for you.

Then we meet Eric Rost, a schlubby Mom’s-basement-dweller who is up to No. 11 in his series of films called “S&Man” (pronounced “Sandman”; get it?). In these, he stalks a woman without her knowledge, gleans extremely creepy information about her, then eventually lures her somewhere and kills her. The M.O. is the same every time. The films are identified by the name of the victim, her hair color, and how she is killed, to aid connoisseurs in finding the volume that suits their needs.

Petty and his crew are unsettled when they converse with Rost. Surely he’s not REALLY stalking and killing these women, or at least not really killing them. (The stalking seems believable.) But why is he so squirrelly when Petty asks him questions? What makes him so shifty about everything?

Berkeley professor Carol J. Clover adds a little insight into sadistic movies as she discusses her book, “Men, Women, and Chain Saws: Gender in the Modern Horror Film,” and Petty meets some devotees at a convention who fit the type you’d expect them to: nerds who feel rejected by pretty girls. The psychology of these movies is hinted at — Rost points out, “I don’t think there’s much market for (movies with) male victims” — but Petty has more devious things in mind.

His film is graphic at times, owing to his inclusion of clips from the movies being discussed, but most of its creepiness comes from its ideas. Do snuff films exist? And with trick photography and sleight-of-hand available to even the most cash-poor filmmakers, could you spot a “real” murder even if you saw it? How hard do you really want to look?

B (1 hr., 26 min.; R, a lot of graphic violence, harsh profanity and some sexual dialogue.)