SIOUX FALLS, S.D. (AP) — A man who stabbed to death a former co-worker who interrupted him during a burglary is set to be executed in South Dakota today barring a last-minute stay.

Charles Russell Rhines brushed off a plea for mercy from 22-year-old Donnivan Schaeffer in the 1992 slaying at a Rapid City doughnut shop. Now 63, Rhines last week unsuccessfully challenged the drug the state plans to use in the execution at the state prison in Sioux Falls.

Rhines argued the drug, pentobarbital, isn’t the “ultra-short-acting” drug he’s entitled to, but a circuit judge ruled it acts as fast or faster than other drugs Rhines cited when used in lethal doses. He’s appealing that ruling to the state Supreme Court.

Rhines has also asked the U.S. Supreme Court to block his execution. He argues that the jury that sentenced him to death had an anti-gay bias, a claim the court declined to hear before. He also argues that the state has denied him access to experts to examine him for cognitive and psychiatric impairments; the state argued that he was examined by mental health experts and found competent.

Gov. Kristi Noem has said she won’t block the execution.

Schaeffer was delivering supplies to Dig ‘Em Donuts where he worked when Rhines ambushed him, stabbing him in the stomach. Bleeding from his wound, Schaeffer begged to be taken to a hospital, vowing to keep silent about the crime; instead, he was forced into a storeroom, tied up and stabbed to death.

Steve Allender, a Rapid City police detective at the time of the killing who is now the city’s mayor, said Rhines’ jury sentenced him to death partly because of Rhines’ “chilling laughter” as he described Schaeffer’s death spasms.