"Killer Joe" is as difficult to categorize at it will be for some to watch.

“Killer Joe” is as difficult to categorize at it will be for some to watch.

You may see the NC-17-rated film referred to as a “dark comedy,” which is more than a little misleading. It’s more of a morality tale (or total-lack-of-morality tale) best described as a sort of “redneck noir.”

Director William Friedkin (“The Exorcist,” “The French Connection”) takes a twisted script from a Tracy Letts stage play and puts it on a slow boil. The end result is sometimes funny, sometimes squirm-inducing. But that’s sometimes a sign of good, challenging filmmaking.

A young Texan (Emile Hirsch) looking to settle a drug debt hatches a plan to have his mother murdered so his sister (Juno Temple) will get a sizable life insurance claim.

He’s gotten a recommendation for Killer Joe Cooper (Matthew McConaughey), a police detective who moonlights as a hitman. It’s a foolproof plan … until it, of course, is not.

Friedkin finds pitch-black humor early before things go really, really wrong. The cast — also featuring Thomas Haden Church and Gina Gershon — is superb, top to bottom.

McConaughey is the revelation, though. His Joe is a cold killer akin to Anton Chigurh in “No Country for Old Men,” but underneath lies an even deeper sadism. Be warned: There are scenes of brutality that are quite hard to stomach here — especially for fans of McConaughey the heartthrob.

It’s a film that will be loved and hated. I wasn’t firmly in either camp, but I won’t soon forget it.