A man convicted of double homicide was put to death Tuesday by lethal injection at the Oklahoma State Penitentiary, telling supporters, "I love y’all," as victims’ relatives looked on—later saying the smile on his face in the death chamber was the same one that tormented them for two decades.

Kendrick Simpson, 45, was convicted of fatally shooting Anthony Jones, 19, and Glen Palmer, 20, after an altercation between Simpson and Palmer at an Oklahoma City nightclub in 2006.

"I love y’all," Simpson said inside the death chamber. "Thank y’all for being here to support me."

Rev. Don Heath, Simpson’s spiritual advisor, read Scripture during the execution, which lasted less than 15 minutes.

Doctors determined Simpson, who became a published poet behind bars, was unconscious about five minutes in, and later pronounced him dead.

Family members of the victims who attended the execution told The Associated Press they were unsettled by Simpson's demeanor in the death chamber.

"The same smile that had been tormenting me for 20 years, he still smiled that same smile laying on his deathbed," said Palmer’s sister, Crystal Allison.

Attorney General Gentner Drummond wrote in a Simpson's last meal Wednesday included a bacon cheeseburger, large onion rings and a strawberry milkshake, according to New Orleans after Hurricane Katrina in 2005, had a diag­no­sis of PTSD result­ing from the col­lapse of access to basic neces­si­ties in the wake of the natural disaster, according to the clemency hearing in January, Simpson apologized for the killings and said he does not make excuses.

"I don’t make any excuses," Simpson said at the time. "I don’t blame others, and they didn’t deserve what happened to them."

Oklahoma's Pardon and Parole Board denied clemency and the U.S. Supreme Court rejected an appeal to block the execution.

Simpson's execution marked the first of the year in Oklahoma, and the second of the year in the U.S.

Ronald Palmer Heath, 64, was put to death in Florida on Tuesday after being convicted of killing a man during a botched armed robbery in 1989.

The Associated Press contributed to this report.