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Some aging filmmakers grow quiet and contemplative in winter. Not William Friedkin. The director of “The Exorcist” and “The French Connection” wants you to know he’s still a gonzo sensationalist, pushing our faces in the lowdown, amped-up hyper-realities of his vision. “Killer Joe” is Friedkin’s second adaptation of a Tracy Letts play — the first was 2004’s certifiably lunatic “Bug” — and it’s as mean as a snake. The nicest person in this Southern-fried pulp noir is the title character played by Matthew McConaughey, and he’s a Texas cop with a sideline in murdering people.

“Killer Joe,” written in 1991, was Letts’s first play, and there’s a lot of Sam Shepard to his snarling, biting clan of trailer-park rednecks. But if Shepard’s work hints at grand themes, “Killer Joe” has engagingly less on its mind. The fun here — and much of it is fun, even if you’ll hate yourself in the morning — lies in watching amoral idiots dig themselves into a hole of their own stupidity.

It’s a toss-up as to who’s dumber: Chris Smith (Emile Hirsch), a layabout in debt to drug dealers; his car-mechanic daddy, Ansel (Thomas Haden Church, with a face like the front grill of a Buick); or daddy’s trashy second wife, Sharla (Gina Gershon). Just to let us know where we stand, Friedkin introduces us to Sharla nude from the waist down. She’s not trying to be sexy. She’s just too lazy to put pants on.

The fourth member of the family is Dottie (Juno Temple), who’s as close to Tennessee Williams’s Baby Doll as Letts can get without copyright infringement. Virginal, dreamy, possibly backward, she’s a faux-jailbait Daisy Mae and the prize in Killer Joe’s eyes.

Joe is hired by father and son to kill Ansel’s first wife — Chris and Dottie’s mother, unseen until very late in the game — for the insurance money, but, to put it kindly, they haven’t thought the matter through. As collateral, Joe decides to take Dottie, and the scene in which he slowly and thoroughly seduces her is the first of two that earn “Killer Joe” its NC-17 rating and stand to shock the shockable. What gives the sequence both its perversity and dramatic heat is the absolute control of the players. Temple, a British actress (and daughter of director Julien Temple), walks an expert line of naivete and lust. But it’s McConaughey who impresses here and throughout the film. It’s not one of the actor’s showboat roles — Joe’s the most restrained, measured person within miles of these characters. Yet you never doubt that he’s absolutely lethal.

The second envelope-pusher is more problematic, since the scene involves one of the female characters getting beaten and forced into an unnatural act with a fried chicken leg that will probably live forever in gutter-cinema infamy. Friedkin and Letts just want to bust our senses wide open here, to make sure that we find the sequence appalling, disgusting, titillating, hilarious, ridiculous. They want us to feel something, which, given the flat-lined state of American movies today, isn’t asking for much. But Friedkin loses control of his tone and lets the scene tip into the merely sadistic. It’s the difference between the wide tableau of a stage image and the fetishistic detail of a movie close-up.

Then again, no one has ever accused Friedkin of behaving properly in his entire career. Visually and thematically, he’s a filmmaker of extremes — not because he’s looking for art out there but because he thinks it’s more honest and lively and real. “Killer Joe” is a vicious bit of sleaze that happens to be smartly written, impeccably shot, and beautifully acted. As far as its maker is concerned, what you do with that is up to you.