Daniel Radcliffe in Horns.

Harry Potter goes over to the dark side in Horns, a nasty little movie from director Alexandre Aja, maker of Piranha 3D and the decent remake effort The Hills Have Eyes.

Danielle Radcliffe plays Ig, who is accused of killing his girlfriend, Merrin (Juno Temple), after an ugly breakup. Not too long afterward, Ig starts sprouting horns out of his head, much to his chagrin. When people see these horns, they behave rather badly—and they have a hard time lying. Ig uses the horns to not only bring out the worst in people, but to start solving the mystery of his lover’s death.

Radcliffe is great here, utilizing a strong American accent and taking advantage of a nice chance to let his nasty side come out. Temple is adorable as Merrin; her story is told in flashbacks, and she leaves no mystery as to why Ig is so messed up after the loss. Joe Anderson is good as Ig’s musician brother, a man strung out on drugs and hiding a few secrets. James Remar, David Morse and Kathleen Quinlan all make their marks in supporting roles.

The movie is rated a hard R, with crazy violence. Mommies and daddies: Don’t let your young kids watch this one, no matter how much they want to see the new flick with the Harry Potter guy.

As a mystery, the movie is a complete failure, because it’s obvious early on who the murderer is. It doesn’t matter, because the film is very strong as a horror comedy. It’s Aja’s most fully realized film to date, and it contains one of Radcliffe’s best performances.

Horns is available via video on demand and online sources including iTunes and Amazon.com. It also opens Friday, Oct. 31, at Cinemas Palme d’Or (72840 Highway 111, Palm Desert; 760-779-0730).