This latest iteration of the “Bad Seed” formula adds a reincarnation element to the tale of a child capable of the most horrible acts, and director Nicholas McCarthy, cinematographer Bridger Nielson and editors Tom Elkins and Brian Ufberg invest the story with a grittily realistic, tense feel that only enhances the impact of the contrasting shock effects, as unfair as some of them might be in context. There’s certainly none of the slickness that Mervyn LeRoy brought to “Seed” in 1956, or Joseph Ruben to 1993’s “The Good Son,” the Macaulay Culkin take on the premise. The grim approach leaves “The Prodigy” genuinely creepy and unsettling, though it’s also extraordinarily ugly and unpleasant, at times even repellent.

Actually, screenwriter Jeff Buhler conflates two rather different concepts—reincarnation and possession by a spirit that has recently become disincarnated—into a rather confused hybrid. He begins with a deliberately disjointed juxtaposition of the birth in Pennsylvania of a son named Miles to Sarah and John (Taylor Schilling and Peter Mooney) with the shooting of a serial killer later identified as Edward Scarka (Paul Fauteux) by a SWAT team in Ohio after his last victim, Margaret St. James (Bittany Allen) has escaped and led the authorities to his lair.

In a quick montage Miles exhibits advanced skills and knowledge that might indicate he’s the reincarnation of a genius. But by the time he’s eight, as played by Jackson Robert Scott, he’s begun to show some disturbing traits. He might be responsible for an accident that befalls his babysitter (Elisa Moolecherry), and he has begun to burble what sounds like gibberish in his sleep. He also appears to be spying on his parents, and on occasion Sarah thinks that his face changes into someone else’s.

After a horrifying incident at the boy’s school, Sarah takes Miles to Arthur Jacobson (Colm Feore), a researcher who believes that the young man’s body houses two souls, the boy’s and that of a dead person. Jacobson ultimately determines that the second soul is Scarka’s, and after still more gruesome episodes Sarah determines to try to save her son from the malignancy that threatens literally to take over his body completely. But doing so will require her to take extreme measures.

“The Prodigy” doesn’t really play fair when it resorts to juxtaposing Fauteux’s face onto Scott’s body in a few scenes, which suggests a shape-shifting element added to the already messy reincarnation-possession mix. For the most part, however, it’s content to engender chills through mood and atmosphere, including an effective use of light and shadow—though when it comes to moments of violence, it certainly doesn’t stint on explicit bloodletting. And it doesn’t hesitate to get really nasty in spots, as when Miles/Scarka threatens an adult with an accusation of child molestation if he doesn’t do as he’s told.

The cast help by playing things straight, without winking at the audience. Schilling and Mooney are both down-to-earth in their increasing desperation, even if some of the choices Sarah and John make will leave you scratching your head over their denseness. Feore contributes his customary smoothness.

The picture’s real ace, however, is Scott, who—with the aid of a bit of makeup—can switch easily from angelic to menacing, though even he has a hard time pulling off the movie’s next-to-last sequence, in which Miles faces off with his mother, a confrontation that ends in a messier version of the close of “Seed”—the novel and play, not the bowdlerized film.

All told, you probably won’t enjoy “The Prodigy”—it’s far too gruesomely mean-spirited for that. But you do have to admire McCarthy’s single-mindedness in presenting it so uncompromisingly, even if the result is sometimes pretty repulsive.