The Holocaust
Nazi Germany murdered approximately six million Jews — two thirds of Europe's Jewish population — along with Roma, disabled people, Soviet prisoners, gay men, and political prisoners, in a program organized with railway timetables, corporate contracts, and civil-service paperwork. It was not carried out by a mob but administered by an educated, bureaucratic, technologically advanced state. That is the fact that has resisted every attempt to explain it away: the machinery of modern government, turned to the industrial killing of civilians.
A belief system rises, spreads, institutionalizes, and then schisms — changing the rules people accept as legitimate.