This post is in response to zorkie's comment
about what makes the Nazis uniquely evil, by a friend of cba's whose area of expertise is the Holocaust.
I concur that this is an important differentiation between the Nazis and Stalin, or the Nazis and Mao. I'm not sure that this it as categorical as that, though. There are other massacres that served only the most trivial ends; and if we count erroneous practical reasons, the Germans had those, too.
First of all, two examples come to mind. Pol Pot's killing of anybody who wore glasses, and thus was an intellectual, wasn't based on reality. Most of us know that glasses don't denote intelligence (except in me), and there wasn't time for any intellectuals, bespectacled or otherwise, to cause serious problems for the Khmer Rouge. This was just killing for the sake of killing, or perhaps to create an atmosphere of terror, which is a pragmatic purpose, if a flimsy one.
The next thing is current Arab hatred of Jews and desire to kill them. I think we can agree that the reason why many Middle Eastern Arabs haven't wiped out the Jews is purely because of lack of opportunity. And they have no pragmatic reason either. In fact Arabs living in Israel are better off and more free than Arabs anywhere else except America and parts of Europe; if I had to be an Arab resident of the ME I'd pick Israel over the rest easily. Only if you concede the points about Jewish terror or Zionist/Western Imperialism can you construe the Arabs as having a pragmatic purpose underlying their murderous Jew-hatred. So they might have a purpose, it's just, again, flimsy, and to more informed minds ridiculous. As was Pol Pot's pretext for killing people with glasses.
Which brings me to my next points: Germans believe they had pragmatic reasons, although they too are absurd and not grounded in reality. More significantly, though, many respectable historians have put forth suggestions as to why so many Germans went to such lengths to kill Jews. Trains and men and supplies that were desperately needed at the front towards the end were assigned to death camps and concentration camps; even when Hitler surely knew that the war was lost, he stepped up the killing rate in a last burst to try and accomplish his goal, and while we can discount Hitler as a lunatic, many other Germans behaved the same way, and they were not all nuts. There are two explanations for this: either they were all nuts, which is demonstrably untrue as many of them functioned just fine before and after the war, and at any rate while some historians contend that Hitler was clinically mad, very few people think all Germans involved were, except, possibly, Goldhagen, and he has actually said that we can't think of Germans as human, and is beyond the pale of serious academic work. The other explanation is that setting the killing of Jews as the highest priority, ahead of even defeating the Soviets and Allies, made some kind of sense according to the Nazi worldview.
There are to me two good (not contradictory, possibly both valid) theories about this. The first is advanced by Omer Bartov, a sabra, on the eastern front and the mobile killings, as compared to the gassings. He shows how the infantry and artillery on the eastern front lived in absolutely appalling conditions, with absurdly high fatality rates, more than half of which were from exposure or disease, not combat. These men were inured not only to death of the enemy but death of their comrades. They were scarcely dressed, undersupplied, and what food reached them was often frozen and inedible. They developed medical conditions that were thought to have been wiped out centuries ago, skin infections that only occur if you don't bathe for months, and so on. Bartov argues that these men in effect reverted to a form of savagery. Further, group loyalty, which was the basis for the Prussian military for some time, no longer existed, as the high death rates meant that units were reconstituted every 4 weeks or so. In such appalling conditions, and without the morale normally generated by the group deployment, the only way to maintain the fighting force was through brutal army discipline. In WW1 about 20 Germans were executed by courts-martial; in WW2 it was over 15 000. They were under orders not to surrender no matter what, on pain of death for them and their families. Bartov shows how this further brutalized the men, and argues that they were first permitted, and then encouraged, to brutalize civilians and especially Jews and partisans (in the first months of the war, the Wehrmacht punished, even jailed, perpetrators of unauthorized looting, rape and murder, even against Jews). This is the best explanation I've read for why individuals, who were not uncommonly brutal or violent in the prior or subsequent lives, did this - as a safety valve, and as a unifying point that provided common purpose among men who were fighting a losing war and didn't have the brotherhood-in-arms so crucial to motivation.
On a wider level, Saul Friedlander, who survived the Holocaust in hiding in France, has to my mind the best explanation. He has defined what he calls "redemptive antisemitism," which was a result of a number of things, some of which Germany had in common with other nations but none of which were all true of any other country. First, there was extreme political instability in Germany due to the Great War, the revolution and botched revolutions, the poverty caused by reparations, and the general decadence and upheaval of the Weimar period. Other countries saw chaos but none the combination of an antinomian culture, crippling reparations and the humiliation of Versailles. Second, Germany was the heir to two different strands of religious antisemitism, Lutheran and Catholic. Most other countries were exposed to only one variety; since Germany was one of the only countries with large numbers of both confessions, Jew-hatred had been a common denominator and something on which both Catholic and Protestant Germans could agree. Third, and this is where you see the jump from extreme and violent persecution of the Jews, bad as it was, to the desire to eradicate them physically and permanently, once the initial successes of the blitzkrieg were replaced by bloody trench warfare, a new ideology crystallized.
Keeping in mind the religious background, the sense of betrayal that was (illogically) harboured toward the Jews for masterminding/profiting from/not serving in the Great War, the hold that racial science had on basically the western world, not by any means just Germany, we now have to consider a Germany facing, it was becoming clear, an insurmountable enemy in Russia. As realistic hopes of victory faded, a German had to resign himself to losing (which was not only psychologically impossible but a threat to ones life from the Gestapo) or else adopt a new understanding of the situation. Combine that with a neo-messianic charismatic leader, and this becomes even more probable. Many Germans at this point in the war (over half of the victims of the Holocaust were killed between March 1941 and February 1942) were convinced that Hitler was a semi-divine leader, the reincarnation of Barbarossa, the rightful heir of the Holy Roman Empire and all the rest. They believed that Aryan blood had a mystical quality, that the Jews were the antithesis of this, and that their own version of Revelations dictated that the Aryans must, at whatever cost, purify themselves and their land of Jewish blood or ideas or influence, and that then they would be saved. Killing of Jews then became the best way possible to ensure a victory.
So, with this twisted witches brew of ideologies and history, with a crazed messianic leader, and with an unwinnable war that ended up claiming almost 2 million German soldiers, did many Germans conclude that the most rational response, the way they could best contribute to the salvation of their Volk, was by killing Jews, as many as possible, as fast as possible. Nuts? Obviously. But entirely consistent with their weltanschauung. Just as Pol Pot killing the myopic and Arabs believing that Jewish curses will make the penises fall off are insane, but in the twisted inner logic make an insane kind of sense.