Something about playing weary hit men, however, attracts him as a director. A decade and a half after Keaton made his debut behind the camera with “The Merry Gentleman,” in which he also starred as a contract killer given a crack at redemption through friendship with an unsuspecting woman, he’s chosen to direct himself again in a similarly subdued story depicting a man of violence shown a pathway out. In the low-key — and regrettably low-energy — character study “Knox Goes Away,” veteran hired gun John Knox (Keaton) faces down the specter of a fatal neurological disease. In the interim, he’ll tend to some unfinished emotional business.

A deteriorating loss of memory is, of course, no asset when one is trained in precision, ruthlessness and traceless escape. So when Knox screws up what should have been a cut-and-dried assignment, leaving three bodies instead of one, he decides to settle his affairs before dementia closes in on him the way he has on countless others. His victims, we hear, are the “deserving” type: traffickers, pushers, that ilk. The movies do love their upstanding hit men — so much easier to like than the mercenary kind. Our antihero isn’t just an ex-Marine, but a former academic who still reads philosophy and classic literature. Learned and lethal, whaddaya know.

Complicating Knox’s exit from a solitary, dangerous calling, however, is a late-night visit from his estranged son Miles (James Marsden), bloodied and desperate, himself having just killed someone in a fit of righteous vengeance. Knox must now add saving Miles from the law to his list of departure errands, requiring an elaborate plan made more difficult by his rapidly worsening condition and a dogged detective (Suzy Nakamura) following the clues from that botched hit straight to his door.

With its condemned-man storyline (à la “D.O.A.”), weekly afternoon hook-ups with a flinty Polish escort (“Cold War” star Joanna Kulig) and the occasional bleat of a mournful trumpet on the soundtrack, “Knox Goes Away” should be noirishly enjoyable hokum. But instead, screenwriter Gregory Poirier’s tribute to an earlier era’s taciturn machismo is more muddled and ludicrous than fleet and clever.

The material also seems to have locked up Keaton’s creative juices instead of loosening them. While he does convey how a certain kind of cagey loner might greet the inevitable, Keaton employs a straight-arrow storytelling approach as a filmmaker, one that never gets beyond the tempo and tone of a mildly moody TV procedural. Perfunctorily photographed by cinematographer Marshall Adams, “Knox Goes Away” may take place in the noir capital that is L.A., but it could just as easily be Anywheresville.

Even harder to reconcile is how little is done with an enviable cast, one in which Marcia Gay Harden as Knox’s ex-wife and Al Pacino as a retired thief and reliable pal named Xavier are, bizarrely, the most muted colors. Pacino in particular looks confused as to why he doesn’t get to simply unsettle us with his usual jolts of energy. Even the unpredictable tension he brought to reading from an envelope at the Oscars was more compelling than anything in “Knox Goes Away.”