Taking a cue from the movie’s soon-to-be-infamous spanking scene between Robert De Niro and Leonardo DiCaprio, someone ought to paddle whoever let Martin Scorsese take three and a half hours to retell “Killers of the Flower Moon.” You could read David Grann’s page-turner — about an audacious 1920s conspiracy to steal resources from the Osage people by murder — in less time, and you’d learn a whole lot more about how J. Edgar Hoover and the newly formed FBI used this case to establish their place in American law enforcement.

Granted, this is cinema legend Martin Scorsese we’re talking about. For years, he fought studio execs telling him what to cut, going head-to-head with Harvey Weinstein on “Gangs of New York” (a movie that probably would’ve been better longer). Now he’s earned the right to tell stories as he sees fit. Trouble is, at 206 minutes (still four shorter than “The Irishman”), “Killers of the Flower Moon” isn’t an epic motion picture so much as a miniseries. Nothing wrong with that, except it’s intended for the big screen — where Apple has committed to release it this fall. Closer to two hours, “Killers” would make a killing, whereas longer than “The Longest Day,” most folks will wait to watch at home.

This is why someone needs to stand up and tell Marty to rein it in. They should’ve done it before he started shooting, since the pace is built in, and Scorsese’s projects don’t compress well after the fact. In its present form, “Killers” is still a compelling true story, one that Scorsese and co-writer Eric Roth shifted from being a standard white-savior detective yarn to a more morally thorny look at how the white culprits plotted and carried out the murders. Stylistically, this feels like a young man’s movie. It’s engrossing from the get-go, the palpable tension methodically echoed by Robbie Robertson’s steady-heartbeat score. But it keeps going and going until everyone we care about is dead, dying or behind bars, with nearly an hour still in store.

Years earlier, the tribe had been forced by the U.S. government to relinquish its ancestral homelands and relocate to undesirable Oklahoma land, where it got rich virtually overnight when oil was discovered beneath its feet. An early scene of that first gusher being discovered recalls “There Will Be Blood,” much as the De Niro-DiCaprio paddling does that film’s bowling-alley finale — although I’m afraid “Heaven’s Gate” is the more fitting comparison: a morally indignant look back at a slow-motion massacre that gets so bogged down in details, it loses the thread.

Scorsese opens on prosperous times for the Osage people, who’d become the wealthiest Americans per capita, thanks to the countless oil derricks that cover their bland land. That made them obvious targets to be exploited. Early on, the director draws a direct line between the Osage Murders and the 1921 Tulsa Race Massacre, referenced via old-timey newsreels — both cases in which white supremacists couldn’t stand to see others prosper, counting on a biased legal system to cover their crimes.

But this isn’t the story of one murder. Taking a page from “Goodfellas,” Scorsese runs through half a dozen suspicious deaths right upfront, dismissed without investigation, including a “suicide” where we see someone shoot an Osage woman through the chest, then restage the scene by placing the gun near her hand. That’s the climate into which DiCaprio’s character, an opportunistic World War I veteran named Ernest Burkhart, moves to Fairfax, Okla., where he soon finds himself participating in the killings. Ernest’s first stop off the train is his uncle William “King” Hale’s place, where the well-connected cattleman (played by De Niro) welcomes him to town, glad to have the perfect patsy.

Ernest doesn’t realize it, but the scheme is already underway. For it to work, King needs his nephew to marry Mollie Kyle (Lily Gladstone), an Osage woman who’s too sharp not to recognize a gold digger, but too trusting to imagine just how sinister her suitor’s intentions may be. Almost right away, her relatives start dying of suspicious causes. One sister succumbs to a strange “wasting disease,” another is discovered with a bullet wound to the back of her head, and the third dies in an explosion so big, it blows out all the windows for a mile in every direction.

No question, these crimes are unconscionable. To make audiences feel the revulsion, Scorsese shoves the victims’ bloody skulls in our faces — except he knows full well that audiences crave “whackings.” In a way that seems almost strategic, given the running time, the murders perversely become a thing to look forward to, carrying viewers through long dry stretches of drama till the next horrific execution. With each death, the family fortunes flow toward Mollie, whose headrights can legally pass to her husband, if she so bequeaths it — all as King had foreseen.