Harry Potter travels to the dark side—involuntarily, and as it turns out only temporarily—in French director Alexandre Aja’s adaptation of the 2010 novel by Joe Hill (the son of Stephen King). The lead role of Ig Perrish represents another attempt by Daniel Radcliffe to distance himself from the persona of J.K. Rowling’s juvenile hero, and in that respect it serves the actor well. Otherwise, however, the picture is considerably less successful.

Perrish is introduced as a pariah in his Pacific Northwest town, widely believed to have brutally murdered his long-time girlfriend Merrin Williams (Juno Temple), whose body was found beside the treehouse in the forest that the two had used as a hideaway since they were children. Flashbacks show how they had met years before, when Ig (Mitchell Kummen) was running about with his older brother Terry (Jared Ager-Foster) and their pals Lee (Dylan Schmid), Eric (Eric McNamee) and Glenna (Laine McNeil). As soon as young Ig saw Merrin (Sabrina Carpenter) and her father Dale (David Morse) in church, he was besotted with her, and their affection had blossomed over the years.

Now, however, the only people who support Ig as the local police, including beat cop Eric (Michael Adamthwaite), build their case against him, are Lee (Max Minghella), a public defender serving as his attorney; Terry (Joe Anderson), a musician; his parents Derrick and Lydia (James Remar and Kathleen Quinlan); and bartender and all-around loose girl Glenna (Kelli Garner). Dale loathes him, and even their priest despises him. It’s no wonder that Ig, followed about constantly by a train of pestering reporters, drinks heavily, or why in one drunken moment he abuses the impromptu shrine set up at the murder site, railing against the God whom Merrin worshipped but allowed all this to happen.

That act serves as the key to a transformation. After a night with Glenna, Ig awakens to find demonic horns beginning to grow from his forehead. He consults a doctor, but the experience reveals that along with the horns comes a dark power: people around Ig begin confessing their darkest, most wicked desires to him, and asking him for permission to give in to them. Initially appalled by the ability, which reveals truths (about his parents, for example) that he’d prefer not to hear, Ig decides to use it to do what the police have been unable to—identify Merrin’s true killer. But there’s some doubt about whether he can manage that without turning fully devilish, something that seems more and more likely as the horns grow, snakes begin congregating around him, and he starts carrying a pitchfork. (It’s a serendipitous touch that his car is a Gremlin.)

This is, to be sure, an oddball premise, and had Hill and adapter Keith Bunin thought it through and treated it coherently, it might have made for a film that was more than weirdly interesting. But they haven’t, and as a result the picture never establishes exactly how Ig’s demonic powers work—sometimes they do and sometimes they don’t, and they seem to increase or subside randomly (though there’s at least the suggestion that their potency is limited against good people—or maybe though protected by religious accoutrements, though the latter certainly doesn’t apply to the priest). The result is an “anything goes” vibe that’s mildly irritating and off-putting.

The other problem with the narrative is that the “whodunit” aspect is frankly a mess. At one time or another suspicion is thrown on virtually every other character, but without much effect, since the guilty party will probably be guessed by most viewers fairly early on—rarely has the “least likely” rule been more egregiously applied. And tying up the plot takes so many outrageous convolutions—as well as plenty of splashy horror-movie gyrations in the final sequence—that in the end you’ll doubtlessly find that the picture works far better simply as a ghoulish comedy than as a thriller. And even though that might be the intent, it doesn’t alleviate the feeling that you’ve really be wasting your time trying to make sense of all the morbid goings-on.

There are nonetheless benefits here. Radcliffe is certainly committed, and getting away from standard leading-man expectations appears to suit him. His scenes with Morse, moreover, capture some authentic emotion that’s lacking elsewhere in the picture. Temple comes across as a trifle bland here, though the sequence in which Merrin breaks things off with Ig and claims that she’s been seeing somebody else (horns, after all, can signify someone who’s been a cuckold as well as a demon) shows her at her best. (Unfortunately, she’s also saddled with one of the movie’s worst lines, at the very start, when she has to ask Ig whether he’s horny—the sort of self-referential joke that must have played better when the filmmakers were thinking about it than it does onscreen, where it carries a nudge-to-the-ribs quality that’s almost painful.)

The others in the cast are generally encouraged by Aja to play things very broadly, which renders even the crassly funny bits (like the one in the doctor’s office) overdone. But the youngsters in the flashback scenes come across nicely, with Kummen in particular contributing a genuinely harrowing near-drowning montage early on. That scene also demonstrates the professionalism of the technical crew, headed by cinematographer Frederick Elmes and production designer Allan Cameron, who make fine use of the locations.

In the end, however, “Horns” is a near-miss, a supernatural romance-cum-whodunit that strains under the weight of its warring elements and winds up an ambitious but unsatisfying jumble, as conflicted as poor Perrish is himself.