In his earlier, naive days, Roger Griffin, a great historian and authority on Fascism, summarized his conclusions about Nazi mass-murder on his website:

Since so many millions were involved in Nazism and the Holocaust, this can’t be explained in terms of madness or pathology.

This is equivalent to saying that because millions of people get cancer--this disease cannot be understood as a form of pathology.

How desperate human beings are to deny profound political sickness. Yes, Nazism and the Holocaust can be understood as madness and pathology. Extreme manifestations of collective madness or psychopathology.

And how we flee from the ugly truth: that madness and pathology are fundamental characteristics of the behavior of human beings in society: that madness and psychopathology are normative.