As the executioner for the US state of Virginia, Jerry Givens put to death 62 people, but after he ended up in jail himself for a crime he says he did not commit, he has become an outspoken opponent of capital punishment.
The 60-year-old African-American worked as a correctional officer between 1974 and 1999, when he was charged with money laundering and perjury and forced to resign.
"In 17 of those 25 years I executed 62 people, I executed 37 by electrocution and 25 by lethal injection," he said, his voice trembling with emotion, on the sidelines of the four-day World Congress against the Death Penalty in Madrid. "It was like a rollercoaster, up and down, because as a correctional officer I prepared inmates to return into society as a productive citizen and as an executioner you take lives."
As the state executioner he would shave the head of convicts facing the death penalty and strap them into Virginia's electric chair or inject them with a lethal mix of drugs.
"We put a cap over your head and send in 3,000 volts to your body, that is gross," Givens said.
He said when he was an executioner he had felt "that person does not deserve to live". "I didn't take full responsibility of this guy being there, I shifted back to the judge, the jury, the family members and himself. I didn't take the whole burden on myself."
His view of the death penalty changed in 1999 when authorities accused him of buying a car with a friend with funds he knew came from drug dealing and charged him with money laundering and perjury.
Givens, who maintains his innocence, was convicted and spent four years in jail. "Then I asked myself: were some of these 62 people treated unfairly like me? I don't know that, but there is a possibility. Were any of them innocent?" he said.
The congress is organised by the French lobby group Together Against the Death Penalty.
About 1,500 people from 90 countries - including high-profile politicians such as French Foreign Minister Laurent Fabius, former death row inmates and family members of people facing execution - are attending.
Souad El Khammal, president of a victims' association founded after the 2003 suicide bombings in Casablanca, said she fought against the death penalty even though she lost her husband and son in the attacks, which left 45 people dead, including the 12 bombers.
"At the time, if I had the possibility to kill the people who took the life of my husband and son, who destroyed my life, I would have killed them with my own hands," she said. "But later I returned to my principles," she said, adding that she and her husband had opposed the death penalty before the attack.