Two roots of Hitlerism and the Holocaust:

Antisemitism: earlier - religious; modern - genetic/ racial (with various theories of racial characteristics and degeneration, some but not all related to modern evolutionism, but in a pseudo-scientific form; eugenics, the systematic breeding of human beings, by force or persuasion, is a related notion, and was advocated by high-profile scientists in Nazi Germany, in America at the same time, and elsewhere.

Extreme nationalism - long-term but, particularly, during and after the second German Reich (1871) of the Hohenzollern monarchs. After 1918, the radical right circulated the Dolchstoßlegende (stab in the back story), saying that, since Germany surrendered even before enemy forces crossed its border, the war must have been lost intentionally through the plotting of leftists and Jews. (But actually Germany, by the fall of 1918, was on the edge of economic collapse, and the peace terms that would have been imposed after a military conquest of the country would have been even more severe than those that were served on Germany - and those were bad enough that it can be said they contributed to the suffering and destabilization that brought Hitler to power.)

The two combined into the Nazi concepts of Volk & Rasse: a nation whose identity was created by the essence of its people, whose traits were inborn and could not be acquired by those of other races (with "race" defined, as it was elsewhere at the time, to include not just differences of color, but even of ethnic background).

And the specifically Austrian and personal factors: Austria, Hitler's birthland, as a particular hotbed of Antisemitism; Hitler's early experiences (decorated WWII combat veteran, alienaiton and failure in Vienna, with its large Jewish population).

The paradox of Germany: progressive Jews much valued it as a home and a culture. France was notoriously more antisemitic, as were Eastern Europe and Austria

Jewish assimilation in response to increasing official toleration and social acceptance, starting in the 18th Century

Jews served eagerly in the German armed forces in the Franco-Prussian War and World War I, with casualty rates and proportion of decoration for service above that of non-Jewish Germans.

Assimilation and philo-Germanism was apparent in new naming customs (Helmut, Siefgried); "The one people in Europe who actually loved the Germans was the Jews."