On Monday, family, friends and colleagues gathered in a small country church for the funeral of the journalist and historian Gitta Sereny. Everyone was united in one thought: we had never known anyone like her.

Those who worked with her, experienced her passion and terrifying energy. It was passion with a purpose. Born in Vienna in 1921, she witnessed a Nuremberg rally, she worked with displaced children after the war and attended many Nazi trials. And for half-a-century afterwards she threw herself into the task of attempting to understand what made human beings do evil things.

Even among the many thousands of volumes on the Third Reich, her two major works stand out as extremely telling documents, enlightening and disturbing. They bring us into prolonged close encounters with perpetrators of terrible deeds and do not let us or their subjects escape. For her book, Into That Darkness, she spent 70 hours interviewing Franz Stangl, kommandant of the Sobibor and Treblinka death camps, picking her way through his excuses and prevarications, confronting him with truths. He was allowed to get away with nothing.

Some said that Stangl should not be allowed to explain anything. But Sereny was a genius at interrogation: forensic, insightful, imaginative and exhausting. At the end of the book, she asks Stangl's wife about her own complicity: what, if she had forced her husband to choose between Treblinka and losing his wife and children, would he have done?

Frau Stangl admitted she did have the power to stop him but had not used it. The day after Sereny finished interviewing him, Stangl died. "He had not committed suicide," wrote Sereny, "I think he died because he had finally, however briefly, faced himself and told the truth." Perhaps, though she may have underestimated what 70 hours being interviewed by her would do to a man.

She allowed him to get away with nothing

Her greatest work - the product of decades of effort - was her biography of Albert Speer, a man who had loved Hitler, built his monuments, used forced labour and escaped the death penalty at Nuremburg by denying that he knew about plans for the Holocaust. Speer was a cultivated and charming man who, during his time imprisoned in Spandau and after, worked rather brilliantly to present an acceptable face to the world.

Sereny got to know him well, they had a long and not unfriendly relationship, a sort of dance of mutual manipulation: he trying to get her to accept his version of events, she trying to extract truths he had never told.

In the end, she succeeded: he confessed he knew about the planned Holocaust earlier than he had ever admitted. If he had told that truth at Nuremberg he would have hanged. "An essential experience that conveys like no other book the qualities of the Nazi elite," said the historian David Cesarani

Last week, the Daily Mail published an article by the journalist Tom Bower. Besides confusing and conflating two Sereny books written 26 years apart and insulting the readers' intelligence by making a debating point of her Austrian accent, he accused her of being more sympathetic to villains than victims. Not just a tawdry article, laced with innuendo, it was a campaign manifesto for stupidity.

Sereny was a passionate hater of cruelty who was brilliant at exploring why people do the evil things they do. She never stinted in her belief that the truth about horrendous men and horrendous deeds can be more complex than it seems.

As an investigator, she was formidable; as an interviewer, she was like a seductress, who lures you in then does the business. Her desire - to understand. Her motto - to understand is not to forgive.