Two very popular misconceptions about the SS ideology must be addressed. First, it was not about “superior” and “inferior” races; it was about an existential war with the “alien race”. It did not matter that the latter was inferior or superior to the “Aryan race”; all that mattered that it was a genuinely existential threat to the German nation and the whole Aryan race – and thus had to be treated accordingly.
Second, by itself, Nazi racism and anti-Semitism were not exactly unusual and definitely not murderous as all governments and political elites of all key world powers consisted of racists and anti-Semites.
The latter was proven beyond the reasonable doubt by the outcome of the infamous Evian conference of 1938 and by general attitude of large and small European and North American nation to the plight of persecuted German and Austrian Jews. As the result, over 200,000 Jews perished in the Holocaust.
The one component of Nazi ideology (and the ideology of the SS) that was the cause of the Holocaust and other mass murders (and other crimes against humanity) committed by the Nazis (mostly by the SS) was their sincere belief in the existential racial war (first and foremost, with the mythical “Jewish race”).
The (perceived) existential nature of this “racial war” inevitably led to the fundamental “the end justifies the means” principle of the Nazi ideology (and thus of the ideology of the SS).
In a genuinely existential war, victory is not everything – it is the only thing. When the war is about the very existence of the German people, of the Aryan race and of the Western Civilization, there can be no moral constraints. The morals are plain and simple – everything that helps to win the war, is good (and moral) and everything that stands in the way to victory, is bad (and immoral).
So after Himmler and other Nazis (incorrectly) believed that the mentally (and sometimes only physically) sick individuals stand in the way to victory in the World War II (by consuming precious food, labor, space and other resources) they made the decision to eliminate this “burden on the German society and German people”. And eliminated – via the infamous “Aktion T4”.
When they decided (incorrectly) that to win the existential war with “Judeo-Bolshevism” (the latter was the existential threat to Germany while the former was not) they had to physically exterminate ALL Jews on German-controlled territories, the Nazis (first and foremost, the SS) immediately put together and (almost completely) executed the corresponding Endlösung der Judenfrage (“The Final Solution or the Final Solution to the Jewish Question”).
Murdering (in gas chambers, by firing squads, “extermination through labor”, etc.) from five to six million (my estimates are on the lower side) of unarmed civilians – men, women and children. In what later became known as The Holocaust (or The Shoah).