The dark secrets of 'Dear Heinrich'

Her father purchased the collection for a "symbolic amount" and Lapa set about trying to establish how it had found its way from Himmler's house in Gmund, in 1945 - where the documents were discovered by American soldiers who'd kept them rather than following protocol by handing them over to the authorities - to Tel Aviv. After a year's research, the paper trail still remained sketchy.

"We will never know for sure," says Lapa. "The three possible options are that Chaim Rosenthal got [the collection] in a flea market in Brussels; or that he got it in a flea market in Los Angeles; or that he got it in Los Angeles from an English couple who were crossing the American-Mexican border."

Lapa's documentary, The Decent One, now uses the material to build a portrait of Himmler in his own words, backed by a wealth of archive footage that deconstructs the image he took to the grave of himself as a decent man, through counterpoint and illustration of the brutal truths that Himmler appears to think were too workaday to detail. By showing the man who oversaw the development of methods to make mass murder quicker and more efficient as someone who was all-too human, Lapa believes that she has made a film that only someone of her generation could make.

Her father's generation is too connected to the events, she says. "The second generation of Holocaust survivors cannot put themselves in a position where they even look at the psyche or something that concerns the side of perpetrators, because they are too close and too traumatised by their parents," she says, explaining that her maternal grandfather was an Auschwitz survivor. In the past, just the idea of an endeavour such as The Decent One might have been misconstrued as an attempt to understand the behaviour of someone like Himmler. This, though, was never Lapa's intention. "We cannot try to understand, and shouldn't try to understand, because it may seem that we're excusing," she says. "And there are no excuses. We shall know and learn and have information in order for it to not happen again. So we shall analyse. That's what I think we should do."

Using immersive sound, compelling footage and the letters and diaries, Lapa and her team take us deep inside a world where the abnormal is normalised and the grotesque becomes the every day.

Gudrun writes about a pleasant day out in the gardens of Dachau concentration camp while Himmler sends items seized from Jews home to his family as gifts, contravening his own proclamation that soldiers must not steal from their victims. At every stage, his image of himself as a righteous and decent man is undermined by the film-makers, in a sort of gruesome version of The Emperor's New Clothes.

"I am doing it because I had the tools to do it," says Lapa. "On the one hand, I had his writing, and on the other hand, historical research and the facts. He is constantly giving us the tools to show him as perverted and corrupted. Besides the mass murderer that he was, he's not even standing up for the decency he's praising so much. At the end of the day, he's also committing suicide. I would have more respect for a character like this that goes all the way with his beliefs. At least stand up in front of the world and stand behind what you praised."

Perhaps most disturbing is the sense that we are beginning to see the history in the film repeating itself. Himmler's dream of German expansionism and the country's return to a time that never was, his ultra-nationalism, hatred of Jews and homosexuals, and the radicalising of values at university, has chilling echoes in IS and the lurch towards extremism in general.

Being Israeli has meant that Lapa has sometimes found people trying to push her into a corner. At a recent Q & A in America, someone asked if she'd made the film "to show that what Israel is doing is like what the Germans did. I tried to not even give that any intelligent reflection," says Lapa. "It was a stupid provocation. I hope that what she meant is, 'Is there a danger today like there was in the '30s and the '40s?'. And the answer is definitely yes. We hear it on the news every day, about leaders that are following the same narrative as Himmler and Germany in the '30s."

Lapa hopes The Decent One can help shine a light on the dangers we face by acknowledging that the Holocaust and the whole Nazi project was the work of ordinary people who made catastrophic choices. Meanwhile, the Himmler papers, which are now stored in a safe deposit box in Tel Aviv, are being scanned for use as educational tools in universities and schools.

'The Decent One' opens in London, April 3.