Walls gave an apology in a brief statement, Alex Lanfranconi, a spokesperson for Gov. Ron DeSantis, said.
"Good evening, everyone," Walls reportedly said. "I appreciate the opportunity to say what’s on my heart. If any of the members of the family are here, I am sorry for all of the things I did, the pain I caused and all of that you have suffered all these years."
The execution started after Walls issued the statement, and the priest remained next to Walls, appearing to pray.
Court records show Walls broke into a mobile home in the Florida Panhandle belonging to Eglin Air Force Base airman Edward Alger and his girlfriend, Ann Peterson, in July 1987.
After they were tied up, Alger managed to break loose and confront the attacker. Walls cut Alger’s throat and shot him in the head as the airman fought back, then moved to Peterson and shot her as she struggled.
The day after the bodies were found, Walls was arrested after his roommate notified police about his odd behavior.
Investigators searched the home and reported finding items from the crime scene. Walls later confessed to the killings.
Following his conviction, DNA evidence connected Walls to the May 1987 rape and murder of Audrey Gygi. He pleaded no contest, sparing him another trial and a potential death sentence.
Walls also admitted responsibility for the killings of Tommie Lou Whiddon in March 1985 and Cynthia Sue Condra in September 1986 as part of a deal with prosecutors.
His attorneys appealed the convictions in state court, claiming he should be disqualified from execution because of his intellectual disability and other medical issues. However, last week, the Florida Supreme Court ruled against Walls.
The U.S. Supreme Court also denied his request for a stay of execution on Thursday afternoon.
Walls woke up at about 5:10 a.m. Thursday, and, according to corrections department spokesperson Jordan Kirland, he remained compliant.
Officials did not say whether any family members of the victims would attend the execution.
Forty-six men have been executed by court order so far in the U.S. this year, and more than a dozen others are scheduled to be put to death next year.
The Supreme Court reinstated the death penalty in 1976, and, since, then the most executions Florida had carried out in a single year was eight in 2014.
Florida has executed more individuals this year than any other state, followed by Alabama, South Carolina and Texas with five each.
Florida’s lethal injections use a sedative, a paralytic and a drug that stops the heart, according to the Department of Corrections.
The Associated Press contributed to this report.