It seemed an ordinary moment, repeated in thousands of schools worldwide. On one side, a shy boy "with puffy, red cheeks" who stammered through a translation test in the principal's office. On the other, a tutor hired by the boy's father to put him through his paces.

But the school was in North Korea, the father was the country's legendary founder, Kim Il Sung, and the boy was his son and future leader, Kim Jong-il. And the relationship between student and tutor would climax in a horrific denouement: the boy grew up to order the execution of the teacher's entire family.

Those are among the milder allegations made by the tutor, Kim Hyun-sik, now 76 and a research professor at George Mason University in Virginia.

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"So many times I've imagined killing him and then killing myself," he writes of his former student in a powerful new memoir in Foreign Policy magazine which pulls back the curtain on the Supreme Leader's background.

The piece, in one of the most influential US publications, is likely to fuel criticism of nuclear-equipped North Korea and embolden the US conservatives who demand military intervention there.

But Professor Kim says he has just one wish: that Kim Jong-il open the country's doors to the freedom and abundance the rest of the world enjoys. Until then, he is determined to tell everyone what he has seen: "A young, innocent boy who turned into a monster, and a country so full of promise transformed into a concentration camp."

The story begins in 1959. The professor, who Kim Il Sung had handpicked to tutor his family in Russian, summoned Kim Jnr, then 17, to take an oral test.

Flustered and "with beads of sweat on his forehead", the boy endured the exam "without ever boasting that he was the son of the Great Leader".

Years later after he had inherited his father's exclusive powers, Professor Kim alleges that the student would order his alma mater blown up to eliminate potential rivals to his own children.

He describes Kim Jong-il as a "rather ordinary student" who excelled at nothing and made few friends. Just months after his test, Kim Jong-il's nervous, diffident demeanour disappeared as he showed off his Russian skills in front of the school's teachers. "As an educator, I was quite gratified," recalls his tutor. But that pride would turn into rage.

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In 1991, a South Korean agent approached Professor Kim while he was in Moscow, saying he could arrange a meeting with the professor's older sister. Like thousands of families, the Kims had been split by the 1950-53 war that had divided the Korean peninsula. The sister, long believed dead, was in Chicago and wanted her brother to join her. "I was overcome with emotion," he writes.

A day later, however, the professor was ordered back to Pyongyang after a North Korean double agent betrayed him. As a trusted insider with intimate knowledge of the ruling family who had been caught talking to the hated South, he knew returning meant one thing. "I would be killed as a traitor." He defected to Seoul and never saw his home, his university or his family again.

Kim Jong-il exacted a terrible price for that betrayal. Professor Kim's wife, daughters and son, their spouses and "even our dear grandchildren" were apparently sent to state gulags and murdered. "To this day, I know nothing of the details of their deaths, of whether they blamed me as they perished." Today, up to 200,000 people are still being held in the North's gulags, according to the US State Department.

The memoir also includes allegations of Nazi-style "cleansing" of the physically disabled and the "substandard".

In one episode, Professor Kim describes how Pyongyang was "purified" of all disabled residents before the 1989 World Festival of Youth and Students, a showcase designed to outdo the Olympics, which had taken place the year before in Seoul. Most victims were "clockmakers, engravers, locksmiths and cobblers" who vanished overnight.

Even short people were not safe, Professor Kim says. The government gave out pamphlets to thousands of people in Pyongyang describing a wonder drug that would raise their height. Instead "they were sent away to different uninhabited islands in an attempt to end their 'substandard' genes from repeating in a new generation. Left for dead, none of the people made it back home."

Bradley K Martin, the author of Under the Loving Care of the Fatherly Leader and a veteran reporter of North Korean issues, says it is true that you do not find handicapped people in Pyongyang. "If you want to see short people who are suffering from malnutrition you have to go outside the capital. I've no reason to doubt this allegation."

Professor Kim's explosive account, extracted from a book published last year in South Korea called A 21st Century Ideological Nomad, comes at a sensitive time. After seven years of sabre-rattling, President George Bush's administration last month removed the North from its list of "terrorist sponsors", signalling that negotiation, not confrontation, has a better chance of luring North Korea in from the cold.

In return, Pyongyang has submitted an inventory of its nuclear programme and pledged to dismantle its weapons. Despite setbacks, its relations with Seoul are also improving. About 70 South Korean companies have set up shop north of the border in the Kaesong Industrial Region, an experimental capitalist project that could bring prosperity to the impoverished nation.

But Professor Kim suggests that the US about-turn is a triumph for the Dear Leader, who concluded in the 1990s that his country, further isolated by the collapse of its sponsor, the Soviet Union, could box above its weight internationally if it prioritised military power. "Today, just as he hoped, Kim Jong-il's vision has been realised," he writes. "[He] has managed to extract resources from wealthier and stronger states by manufacturing crises and generating international instability. His brand of nuclear blackmail is a virtual guarantor of bottomless international aid for the world's most militarised society."

The Foreign Policy article also details a secretive wing of the North's state apparatus called Bureau 3, charged with planning operations against South Korea, including, he claims, the planned assassination of its president and his entourage during a visit to Burma in 1983. The bombing killed 17 South Korean cabinet members, but failed in its main objective.

The account reveals how children of the party elite are sent to exclusive schools and fed rice, meat, fish and eggs while ordinary children get cornmeal and soybean soup. The privileged young do just a few days mandatory farm work a year, unlike their poorer counterparts, who average between 60 and 90 days.

The leader will not allow graduates from his old school into his inner circle. "Those who have known Kim Jong-il since youth are bound to see him as human – not the centre of a god-like cult of personality," Professor Kim explains. He says he remains optimistic that the North will change and would love to meet his former student for a final lesson. "I, who became a university professor thanks to his father; I, who travelled to Russia, Seoul, and now Washington – I no longer loathe him. I pity him. Even though he killed my family, I have already forgiven him."

Dictators in class

Robert Mugabe

Born in 1924 in what was then Rhodesia, Mugabe had a solitary childhood. His father abandoned the family when Mugabe was 10 and he came under the influence of an Irish Jesuit who fed him tales of the Irish struggle for liberation from the British. As a young boy he worked hard at school but avoided other children, preferring to go hunting alone. He left to attend university in South Africa, where he was introduced to nationalist politics.

Adolf Hitler

Born in Austria in 1889, Hitler had a difficult youth, during which he was beaten almost daily by his father. Despite showing great potential at his religious school, he disappointed his mother by deciding against becoming a priest. He moved to Vienna aged 18 to pursue his dream of becoming an artist. After two failed attempts to gain entry to an art academy, he tried architecture instead, but ran out of money. Almost friendless, he scraped a living by doing paintings for tourists. Rejected by the Austrian army, he joined the German army in 1914.

Mao Zedong

The eldest son of a prosperous peasant family, Mao was born in 1893 in Hunan province, central China. His father was able to afford his school fees, although he was required to help out on the family farm from a young age. He left school aged 13 to work full-time for his father,but intellectual curiosity eventually drove him to seek out new teachers in a neighbouring town. Upon completion of his schooling, he went to Peking (now Beijing) to study at Peking University.