Brian Dorsey, 52, shot Sarah and Ben Bonnie after they agreed to help him with debts he owed to drug dealers.
A Missouri man who murdered his cousin and her husband in 2006 has been executed by lethal injection despite clemency appeals by prison staff.
The couple’s four-year-old daughter was at the home during the fatal attack.
He was pronounced dead at 18:11 local time (23:11GMT) on Tuesday, according to a news release from the Missouri Department of Corrections.
Advocates for Dorsey argued that he had reformed during his stay in prison and that he did not have adequate legal defence during his trial.
The US Supreme Court rejected two bids to delay the execution. The governor also rejected a request for clemency, saying in a statement on Monday that Dorsey “punished his loving family for helping him in a time of need”.
Missouri Governor Michael Parsons also cited prosecutors’ claims that he sexually assaulted his cousin’s body after killing her. Lawyers for Dorsey reject the allegation, saying it was never proven at trial.
Dorsey turned himself in to the police three days after the attack. He confessed and pleaded guilty.
He is the first inmate executed by Missouri this year, after four people were put to death in the state in 2023.
Lawyers for Dorsey argued that he had become a model inmate in the last 17 years, one who worked as a prison barber giving haircuts to staff, and was remorseful for his crimes.
They said he was suffering drug-induced psychosis at the time of the attack, two days before Christmas, and that his lawyers faced a financial conflict of interest during trial.
At the time, the Missouri Public Defender Office’s paid court-appointed lawyers a flat fee of $12,000 (£9,500) per case. A low sum, given the thousands of hours normally needed in capital justice murder cases.
They argue his lawyers rushed him to take a plea deal that did not ensure that he would not be given a death sentence.
Former Missouri Supreme Court Justice Michael Wolff wrote to the governor saying the payment system, which has since been changed, has “undoubtedly influenced everything”.
Mr Wolff wrote that executing Dorsey “will dishonor our system of capital punishment”.
A petition signed by more than 70 correctional officers argued his sentence should be commuted to life in prison due to good behaviour.
“The Brian I have known for years could not hurt anyone,” one officer wrote in the petition. “The Brian I know does not deserve to be executed.”
In his final statement before his death, Dorsey again apologised to his victims and their relatives.
“To all of the family and loved ones I share with Sarah and to all of the surviving family and loved ones of Ben, I am totally, deeply, overwhelmingly sorry,” he wrote.
“Words cannot hold the just weight of my guilt and shame. I still love you. I never wanted to hurt anyone. I am sorry I hurt them and you.”
Missouri’s execution protocol allows for “surgery without anesthesia” if the typical process of finding a suitable vein to inject the lethal drug doesn’t work, lawyers for a death row inmate say in an appeal aimed at sparing his life.
Brian Dorsey, 52, is scheduled for execution Tuesday for killing his cousin and her husband at their central Missouri home in 2006. His attorneys are seeking clemency from Gov. Mike Parson and have several appeals pending.
A federal court appeal focuses on how Missouri injects the fatal dose of pentobarbital. The written protocol calls for insertion of primary and secondary intravenous lines. But it offers no guidance on how far the execution team can go to find a suitable vein, leaving open the possibility of an invasive “cutdown procedure,” Dorsey’s attorneys say.
The procedure involves an incision that could be several inches wide and several inches deep. Forceps are used to tear tissue away from a vein that becomes the injection point.
“It’s surgery,” said Arin Brenner, a federal public defender and one of the attorneys representing Dorsey. “It would be surgery without anesthesia.”