The Nazi death camp at Auschwitz was liberated 70 years ago today, but the world still struggles to understand the minds of those who committed the atrocities there. Laurence Rees, writer and producer of a major BBC TV series marking the event, has interviewed war criminals from German, Russian and Japanese camps and he explains why many of the former Nazi soldiers he met had a different mentality from the others.

What sort of man was capable of creating the site of the largest recorded mass murder in history, where acts of atrocity were everyday occurrences? Perhaps someone like Amon Göth, commandant of Plaszow labour camp in Poland (memorably portrayed by Ralph Fiennes in the film Schindler’s List), an irrational, sadistic monster utterly different from the people you encounter in everyday life.

But if you imagined such a person was commandant of Auschwitz, then you’re wrong. According to his interrogator at the Nuremberg war trials, Whitney Harris, Rudolf Höss appeared “normal”, “like a grocery clerk”. Prisoners who came across him at Auschwitz confirmed this view, adding that Höss always appeared calm and collected. There is no record of him ever personally hitting – let alone killing – anyone at the camp.

Höss lived with his wife and children in a house just yards from the crematorium in Auschwitz main camp, where some of the earliest killing experiments were conducted using the poisonous insecticide Zyklon B. During his working day Höss presided over the murder of more than a million people, but at home he lived the life of a solid middle-class German father and husband. It is this apparent normality that makes Höss a more terrifying figure than an unhinged brute like Göth, and compels us to try – in so far as it is possible – to understand him and the circumstances that made his murderous career possible.

Like most ardent Nazis, Höss’s character and beliefs had been shaped by his reaction to the previous 30 years of German history. Born in Baden-Baden, in the Black Forest, to Catholic parents in 1900, Höss was affected in his early years by a series of important influences: an overbearing father; his service in the First World War where he was the youngest NCO in the German Army; his sense of betrayal at the subsequent loss of the war; his service in the paramilitary Freikorps in the early 1920s in an attempt to counter the perceived Communist threat on the boundaries of Germany, and involvement in violent right-wing politics that led to a two year imprisonment.

Many other Nazis were moulded in the same way, not least Adolf Hitler. Son of a domineering father, nursing his violent hatred of those he felt had lost Germany the war in which he had just fought (and during which, like Höss, he had been awarded an Iron Cross) Hitler tried to seize power in a violent putsch in the same year as Höss was involved in a politically inspired murder.

For Hitler, Höss and others on the Nationalist Right, the most urgent need was to understand why Germany had lost the First World War and made what they felt was such a humiliating peace: and in the post-war years they believed they had found the answer....