Tuesday night, Oklahoma tortured a man to death. At 6:23 local time, a doctor began to inject Clayton Lockett with a sedative. Seven minutes later, convinced Lockett was sedated, the doctor then began to inject the second and third drugs in the lethal cocktail that were supposed to end Lockett’s life. But Lockett “began to twitch and gasp” after having been declared unconscious. He called out “man” and “something’s wrong.” He then “struggled violently, groaned and writhed, lifting his shoulders and head from the gurney before the blinds to the [execution] room were lowered 16 minutes after the execution began.” The doctor “intervened and discovered that ‘the line had blown,’ said the director of corrections, Robert Patton, meaning that drugs were no longer flowing into his vein.” (Some outside experts have cast doubt on that explanation.) At 7:06 p.m, Lockett — convicted of shooting a 19-year-old woman and watching two accomplices bury her alive — died of a heart attack in the execution chamber. A second scheduled execution, of child-murder Charles Warner, was postponed. As abhorrent as Lockett’s crime was, his horrific death is just the latest in a long line of examples of why the death penalty has no place in the United States.

Disgracefully, this botched execution was entirely predictable. Since 2011, when the makers of the sedative sodium thiopental, formerly the first drug in the three-drug combinations that were used in almost all lethal injections, stopped producing it, states have been scrambling to fill the gap — with more questionable drugs and sources. As The Post’s Mark Berman reported this year, “the first four executions [of 2014] were carried out using four different combinations of drugs. ‘That certainly smacks of an experiment,’ said Richard C. Dieter, executive director of the Death Penalty Information Center.” And that experimentation has had stomach-churning results, most notably in the case of Dennis McGuire, killed by Ohio in January: “Horrified witnesses watched as the 253-lb McGuire ‘repeated cycles of snorting, gurgling and arching his back’ and appeared to ‘writhe in pain,’ according to a subsequent lawsuit filed by his family.” McGuire’s execution used the same sedative, midazolam, that Oklahoma used Wednesday, with similarly awful results.

And this wasn’t the first execution-gone-wrong in Oklahoma this year. On Jan. 10, Michael Lee Wilson was executed using a cocktail including pentobarbital, a less-effective substitute for sodium thiopental; witnesses report he cried, “I feel my whole body burning,” suggesting the drug wasn’t working. Oklahoma admitted that the pentobarbital used was bought from a compounding pharmacy it refused to identify. Compounding pharmacies are poorly regulated, even though contaminated pentobarbital can lead to excruciating deaths. (“Experts say it can feel as though the insides of a person’s veins are being scraped with sandpaper.”)

Oklahoma corrections officials were troubled enough to switch to a new drug combination for Lockett and Warner’s executions. “The only known use of this drug combination for executions was in Florida in 2013,” wrote Mother Jones’s Stephanie Mencimer before the executions, “but Florida used five times the dose of midazolam that Oklahoma plans to use, meaning Lockett and Warner will essentially be human guinea pigs.” Understandably, the pair’s lawyers wanted to know more information about the sources of each drug, their effectiveness and other details that would determine whether the combination violated the Eighth Amendment’s prohibition on cruel and unusual punishment. When Oklahoma — like a number of other states — refused to release more information, the lawyers appealed, all the way up to the state supreme court. But after the court ordered a stay on the two executions, the state’s Republican governor, Mary Fallin, refused to recognize the court’s stay, and Republican state Rep. Mike Christian introduced impeachment proceedings against the five justices who voted for the stay. So much for separation of powers. Cowed, the court reversed its decision two days later, leading to the awful scene that played out Tuesday.

Yes, Lockett and Warner’s crimes were utterly heinous. But so was this state-sponsored killing, perhaps even more so in light of Oklahoma Republicans’ bloodthirsty rush to execute Lockett and Warner. We have known for years that the death penalty is “cruel and unusual punishment.” We know that the drug cocktails used in lethal injections were designed to be “visually palatable” at the expense of more effectively preventing excruciating pain. We know that the death penalty is frequently administered in a racially biased fashion. And we know that, as reported this week, “about one in 25 people imprisoned under a death sentence is likely innocent.” It is long past time for the United States to end this barbaric practice.