This question isnt exactly easy to answer. The question of how so many people became complicit in an ultimate scheme to murder so many others is a question repeatedly raised by historians and something most people familiar with this era struggle with.

A number of reasons have been elucidated, each with varying levels of explanatory power. I will do my best to explain them here, and hope I have given a good answer to this difficult question.

1. Ideology and Antisemitism

Shortly after Hitler seized power in 1933 he began the process of Gleichschaltung, or Nazification. Every institution of the old Weimar Republic was co-opted for the Nazi regime's benefit to establish totalitarian control over every aspect of German society. The Reichstag Fire Decree and the Enabling Act were just the first in a wide series of measures the Nazi regime used to consolidate complete power and authority over Germany.

Along with it came widespread anti-semitic propaganda. The German people still felt the national humiliation of the defeat in WW1 and the Treaty of Versailles, and Jews became the convenient scapegoat for all of Germany's failures.

This propaganda was ubiquitous in every level of German society and had a definite influence upon the younger generation growing up under the Nazi regime. Growing up they had witnessed the rampant violence and instability, the street fighting and communist and nationalist uprisings of the Weimar era. To them Hitler was a figure, a voice of stability and strength that stood up against the enemies of Germany both within and without.

Not only that, those who displayed firm commitment to Nazi ideals were often promoted and given rank, while those who openly criticised the regime were demoted and excluded. This had the effect of ensuring devoted Nazis were the ones gaining influence and power and prestige in society.

2. Exclusion of undesirables

Along with the rise of the Nazi regime came harsher and harsher anti-Semitic laws (like the Nuremberg race laws). Jews were increasingly excluded from German society as a whole, made increasingly invisible. Many fled overseas, others stayed. When oppression was meted out upon a minority seldom seen, it was too easy to be apathetic and turn a blind eye to it.

But the vast majority of Jewish victims of the Nazi regime werent German Jews. They were the Jews of Eastern Europe and the Balkans, in Poland, Greece, Yugoslavia and the Soviet Union. Many of these Jews were traditional, orthodox Jews, whom dressed conservatively, spoke Yiddish and "stood out" amongst the wider population as a whole.The widespread use of propaganda gave many Germans the impression that these Jews were a foreign "other", an alien presence that did not deserve and did not belong to the soil of the lands they now occupied.

3. Peer pressure, obedience to authority and progressive desensitization

In my opinion, this is the largest, most important factor. (And hence I will go into greatest detail about it) This issue was extensively explored in Christopher Browning's seminal work, Ordinary Men. Much of the book chronicles the true story of Reserve Order Police Battalion 101. The battalion was largely comprised of older men from Hamburg, one of the German cities well known for its strong left wing leanings. Many were from working class or blue collar backgrounds and many had left wing or social democratic leanings, and remained so even after their enlistment. As such they were among the men most ambivalent to Nazism ever fielded. However, in July 1942, the battalion commander Major von Trapp were given a task by SS Police Chief Odilo Globocnik. They were ordered to clear the village of Jozefow (in the Lublin district of occupied Poland) of Undesirables (ie Jews). In effect this meant they were to execute every last Jew in Jozefow.

Major Trapp was deeply shaken by this, but he read aloud the orders to his men anyway, and gave them a first chance of stepping out. Only about a dozen men out of 500 in Reserve Order Police Battalion did, at first. Later, more men actually abstain from the killings, but many others carried out the orders anyway, rounding up the Jews, bringing them to pits in the forest and shooting them. The operation as a whole was a mess, with many of the police officers underfulfilling their quotas, missing their shots or being so revulsed by their actions that they were no longer able to continue. But in the end many, many Jews died at their hands.

Browning estimates that of these men, about 20 percent actively participated, about 60 percent were reluctant executioners, and about 20 percent abstained from the killings.

We can see that, despite the fact many of these men werent fanatical Nazis, many in the end chose to carry out the massacre anyway. Those who refused to carry out their orders were shamed and insulted, but they werent actually punished for refusing.

A number of factors have to be taken into account. First, the orders were given from the "highest authority", according to battalion commander Major Trapp. Second, in Trapp's recital of the orders, the Jews were painted as an existential threat, the reason why Allied bombing was targeting German cities and killing innocent Germans.

Browning points to factors, like peer pressure, obedience to authority and the desire not to be "weak" and the biggest contributors to the reason these men chose to kill in the end. If you didnt participate, someone else had to shoulder the burden of killing these Jews and stain their hands with blood. You were making it worse on everyone else in the team. Were you too weak to "do what needs to be done"?

Not only that the fact these orders came from above also, in the eyes of the killers, absolved them of some of the moral blame. It wasnt their compulsion to kill Jews, they were ordered from the highest authority, and they being policemen had to obey.

The book goes on to describe, in detail more actions that the battalion took part in after the Jozefow massacre. The battalion was divided into companies and carried out even more massacres, such as the Lomazy massacre, and the deportation of Jews to Treblinka.

We can see that the men of Reserve Order Police Battalion 101 were increasingly desensitised to the killings. It became increasingly routine, and they became increasingly numb. They could also now rely on "Hiwis" (Red Army prisoners of war recruited into Auxiliary Police units) to outsource their killings this time.

4. distortion of the moral reference frame

Harald Wezer and Sonke Neitzel's book Soldaten delve into detail on this. The book is written based on source information from secret eavesdropping protocols on German army prisoners of war held in British and American prisoner camps. The German POWs (shockingly) often casually described war crimes they witnessed (or committed), without reference to moral outrage or indignation. In their eyes it was often justified, as being part of the broader struggle against partisans, or as revenge and reprisal against the killings of German soldiers. Some of them went into detail about witnessing Jews being rounded up and shot, or in the case of one Luftwaffe pilot during the invasion of Poland, themselves dropping bombs onto civilians casually, and feeling more sorry for the horses caught in the blast afterward.

Rarely did German POWs ever talk or dived deeply into Nazi ideology. More often they would boast about achievements attained or medals gained. (In the case of Kriegsmarine sailors they would give their estimates of tonnages of shipping sunk, in the case of German Heer servicemen they would talk about their combat experience and how they took down their enemies)

It became increasingly clear that in the world of these soldiers, there was little distinction between military and civilian targets. In the context of the harsh war they were facing, it was justified if it could bring the war to a quicker close.

In the extensive military culture and social engineering schemes of the German Army and the SS, their reference frames to what was considered morally acceptable was heavily distorted. Acts that would constitute war crimes were condoned, even sanctioned and ordered. And an internal justification scheme was formed in the minds of many soldiers. Jews were seen as an existential threat to Germany’s survival. They were partisans that attacked and killed good Germans behind their lines.

Pre-assigning moral blame and guilt to Jews enabled the perpetrators to carry out their actions.

5. the divison of culpability

The Holocaust could not have been carried out without the systematic planning, co-ordination and implementation of numerous schemes concocted by the multiple bureaucracies of the Third Reich. From the procurement of the cyanide from IG Farben to the scheduling of trains deporting Jews to the extermination camps, this all spoke of systematic planning and execution.

These orders were carried out by bureaucrats, middlemen. Many didn’t personally witness the killings, but played a facilitative role in its implementation. As with almost every other branch of the German government by this time, it was heavily Nazified as well.

When people are reduced to numbers on a page, or when their deaths were never seen and heard, it was too easy to feel like merely a cog in a greater machine. To feel personally, not responsible for their death.

Conclusion

Despite these explanations I still feel that it is difficult to accept how the Nazi regime could carry out such an action. It resulted in the near extinction of European Jews and deeply upturned societies as a whole. And it certainly has ramifications. If the majority of these perpetrators were “ordinary men”, are we ordinary people capable of genocide too?