Kwon Hyok is one of about 4,000 North Korean defectors living in Seoul, South Korea.
Locations of secret prison camps, or Gulag, are marked in black
Most escaped because of hunger, fear, torture, imprisonment or a simple hatred of the regime.
But Kwon Hyok is not one of those. In 1999 he was a North Korean intelligence agent stationed in Beijing when he was persuaded by the South Koreans to defect.
Six years before, in 1993, Kwon Hyok says he was Head of Security at prison camp 22 in Haengyong, an isolated area near the border with Russia.
Camp 22 is one of a network of prisons in North Korea modelled on the Soviet Gulag where hundreds of thousands of prisoners are held.
Most of them have been charged with no crime. They are there because of the "Heredity Rule".
"In North Korea, " Kwon Hyok explains, "political prisoners are those who say or do something against the dead President Kim Il-sung, or his son Kim Jong-il. But it also includes a wide network of next of kin. It's designed to root out the seeds of those classed as disloyal to North Korea."
In prison, says Kwon Hyok, "there is a watchdog system in place between members of five different families. So if I were caught trying to escape, then my family and the four neighbouring families are shot to death out of collective responsibility."
Torture, he says, was routine. "Prisoners were like pigs or dogs. You could kill them without caring whether they lived or died.."
"For the first three years" he explained " you enjoy torturing people but then it wears off and someone else takes over. But most of the time you do it because you enjoy it."
But Kwon Hyok had something else he wanted to tell.
He says he witnessed chemical experiments being carried out on political prisoners in specially constructed gas chambers.
"How did you feel when you saw the children die?", I asked.
His answer shocked me.
"I had no sympathy at all because I was taught to think that they were all enemies of our country and that all our country's problems were their fault. So I felt they deserved to die."
There have been many rumours of human experimentation on political prisoners in North Korea. But never has anyone offered documentary proof. Until now.
In Seoul I met Kim Sang-hun, a distinguished human rights activist.
He showed me documents given to him by someone else completely unrelated to Kwon Hyok. He told me the man had recently snatched them illicitly from Camp 22 before escaping.
Kim Sang Hun is convinced the documents are not forgeries
They are headed Letter Of Transfer, marked Top Secret and dated February 2002 . They each bear the name of a male victim, his date and place of birth. The text reads: "The above person is transferred from Camp 22 for the purpose of human experimentation with liquid gas for chemical weapons."
I took one of the documents to a Korean expert in London who examined it and confirmed that there was nothing to suggest it was not genuine.
But I wanted to run a check of my own with Kwon Hyok. Without showing him the Letter of Transfer, I asked him very specifically, without prompting him in any way.
"How were the victims selected when they went for human experimentation? Was there some bureaucracy, some paperwork?"
"When we escorted them to the site we would receive a Letter of Transfer," he said.
Sadly, as long as these reports continue from defectors, and as long as the North Korean government continues to deny all allegations of human rights abuse, while refusing to allow access to its prisons, such allegations cannot be dismissed or ignored.
Access to Evil was broadcast in the UK on Sunday, 1 February, 2004 at 2100 on BBC Two.