I had a wonderful childhood till 1945," says Niklas Frank, describing a life that sounds almost magical. He warmly recalls playing hide-and-seek among the "wonderful monuments" to ancient kings and queens in the cathedral of Wawel Castle in Krakow, Poland, where his family resided for half of the year, and visiting nearby parks with his beloved nanny Hilde. The family had the run of another castle at weekends, and would often return to their house by a lake in Upper Bavaria, where they'd fish, swim and ski.

Wawel Castle, though, was Niklas's father Hans's "kingdom". His mother, Brigitte, called herself Queen of Poland. The Franks weren't royalty, however, but a family in the top echelon of the Third Reich. In 1939, the year Niklas was born, Hitler had made his lawyer father Governor-General of Nazi-occupied Poland, a role that saw him implement the Final Solution with such ruthlessness, that he became known as the Butcher of Poland.

Hans was arrested in May 1945, four days before the end of the war, and put on trial at Nuremberg. Niklas had been protected from the truth about him; bloody reality now poured in. "When I saw the first pictures of corpses in newspapers in 1945, with the word Poland underneath, I knew there was a connection," he says. "My father was detained, so I knew he was responsible for these corpses."

Hans was found guilty of mass murder and hanged. Today, Niklas carries around the last ever photograph of his father, taken post mortem, as a reminder of his criminality. He once described him as a "typical German monster", but now corrects himself. "Monster was the wrong word. He was very educated, he knew by heart and brain what he was doing. Monster is an excuse. A monster is not responsible."

What did he mean by "typical German"? "A typical German always follows orders," says Frank, sneeringly. "If you are ordered to kill someone, you say: 'It's orders, I have to fulfil this task.' They forget their humanity and follow the order, not thinking that this is a crime."

Despite overseeing the murders of millions of Jews, Hans wasn't antisemitic, he insists. "He cursed the French people in his private diary, but there's not a single antisemitic sentence. If Hitler had said the French are the ones, he would have made sure they would all be killed."

Niklas acknowledged his father's guilt from the moment he found out what he had done, and has hated him ever since. This brings him into conflict with an old friend, Horst von Wachter, in the new documentary My Nazi Legacy, when Horst won't accept - under pressure from Niklas and the human-rights barrister Philippe Sands - that his own late, Nazi father, whom he still loves, was responsible for mass murder as the Governor of District Galicia.

Acknowledging what Hans did has kept Niklas sane. "My wife would tell you: 'He's not crazy. He never needs therapy,'" he says, laughing. Nonetheless, did finding out that he was the son of one of the Nazi era's biggest criminals make him look at himself differently? "No," he replies.

"I said to myself, 'I enjoyed [my childhood] because I didn't know better. So it's okay, I don't have to feel ashamed that I was very happy then.' So it was never about shame with me; it was always that I became furious."

This fury exploded onto the page in a controversial 1987 memoir, The Father: A Settling of Accounts (published in English as In the Shadow of the Reich). A bitter J'accuse, the book shocked readers with its vituperative language and shattering honesty. Niklas had found none of the latter in his father, although he did find some in his mother.

In a letter he discovered from Brigitte to a friend, she'd written: "When I'm looking back, we really were gruesome, without any mercy." Hans, on the other hand, "Never did it in such an honest way," says Niklas. "He admitted his guilt at Nuremberg but it was just a trick. His lawyer asked him, 'Have you ever seen anything to do with the extermination of the Jews?' And, after a short pause, he said, 'Yes.' And then he changed it . . . He said: 'A thousand years will pass and not take away this guilt from Germany.' So what about his personal guilt?"

For decades, Niklas has been searching the archives for something in Hans's favour - an incidence of his saving someone, for example - but he hasn't discovered anything yet. His mother also did nothing to help Jews, even though she often did business with them before the war and after the creation of the Krakow ghetto. "She never saved one of her partners. Never," says Niklas, glumly.

She wasn't ignorant of their fate; living and working in Wawel Castle made that impossible. "Everybody knew each other, everybody talked to each other, a lot of SS people were there, all the guards; there was a lot of sexual relationships between all the secretaries and the SS." He was told that his mother's lover (whom Hans mistakenly believed was Niklas's father for a while; nor is he Hitler's godson, as often reported) had once been invited by someone in the SS to come on a Sunday to shoot Jews. "Unbelievable," he exclaims.

Hannah Arendt talked about the "banality of evil" while covering the Eichmann trial, but Niklas thinks this is too simple. "For me, it's not a very clever sentence," he says. "Even the worst atheist has the Ten Commandments in their brain. It's our humanity. So there is nothing banal about this period in Germany. Everyone knew [what was happening] and they [still] decided to go with it. I can't forgive this."

Antisemitism is on the rise again and he is worried about what might happen were a charismatic Far Right figure like Heinz-Christian Strache in Austria to appear in Germany. "We committed the worst crimes that were ever committed around the world, so we should know better. But do we? I'm really afraid about what's going on. Still, our economy is good. But if our economy breaks, not just for one year, let's say five years, they will hunt [people] down again. Maybe the Jews, or whoever we make responsible."

People need to be brave and speak out more, he says. In researching his father, he realised that he was "a big coward. . . And I found out what a big coward I was myself during my working life [as a journalist for Stern magazine], and it helped me not be as cowardly as my father."

He wrote The Father to break the oppressive silence in Germany about what the previous generation did in the war, and to show that "you have to be tough and you have to ask your parents". He now takes every opportunity offered to him to speak in schools about his father, and to warn about what can happen when civil society breaks down, in the hope of helping to prevent it happening again. "We experienced that if you don't have any civil courage, starting with the little things, it leads to building up extermination camps," he says, passionately.

"I am a chauvinist when it comes to German crimes. I don't want to compare with the behaviour of other people. Everybody knows how right-wing France is. Everybody knows about the Italian fascists. Everything is connected, especially in the European Union, and you have to be careful."