Clayton Lockett, a convicted murderer, was supposed to die by lethal injection Tuesday. Instead, he died of a heart attack 43 minutes after receiving the state-administered injection.

Lockett’s guilt was not in doubt, and the death penalty is popular in Oklahoma. Nevertheless, his case demonstrates how capital punishment is becoming impossible to administer. Even its supporters must concede as much.

The difficulties in Lockett’s case, and in a similar botched execution in Ohio in January, are the result of prisons experimenting with new lethal drug combinations now that European manufacturers won’t (or, since 2011, can’t) supply the drugs that worked in the past.

If Americans are uncomfortable with a criminal justice system that results in a man thrashing on a gurney, straining to lift his head and calling out as not-quite-lethal drugs enter his bloodstream, as Lockett did, they are unlikely to be reassured by the alternative killing methods states are considering: gas chamber, electric chair, hanging. Those methods, after all, were largely abandoned in the 1980s to make capital punishment more palatable and to avoid violating the Constitution’s prohibition against cruel and unusual punishment.

Capital punishment, which is practiced by no other advanced nation besides Japan, might be defensible if it deterred crime better than prolonged imprisonment. But there is no convincing evidence that it does.

Meanwhile the evidence is mounting, in Oklahoma Tuesday night and elsewhere, that there is no humane way to put people to death. This fact may not much bother supporters of capital punishment. But no American should support a government when it knowingly inflicts pain and suffering.