After he was strapped into the death chamber in Huntsville, Texas, Billy Frank Vickers, 58, admitted for the first time that he shot Phillip Kinslow during a botched robbery in 1993.
"It was nothing personal, I was just trying to make a living," Vickers said.
Vickers also took responsibility for more than a dozen other killings.
He said there were "several more that I had done or that I had been a part of, and I'm sorry but I am not sure how many. There must be a dozen or 14, I believe, all total".
He mentioned no names, except in the case of a former Texas oil millionaire who was accused and later acquitted of killing his stepdaughter in 1976.
"One I would like to clear up is Cullen Davis where he was charged with shooting his wife," Vickers said, without elaborating or taking clear responsibility for the killing.
Cullen Davis was accused of killing his second wife's 12-year-old daughter at his Fort Worth mansion. Priscilla Davis, his second wife, was wounded and her boyfriend was killed. Cullen Davis was later acquitted of murder-for-hire charges in a separate case.
Jack Strickland, a former prosecutor who worked on the Davis case, said he had never heard of Vickers and doubted he was involved in the shooting. Vickers died at 6.21pm on Wednesday night, six minutes after the lethal dose began.