Social phenomena — both good and bad — cannot happen without cooperation — that is, without people doing things uncoerced. This is especially clear with bad phenomena. How much havoc could a Stalin, a Hitler, a Mao Zedong, or a Pol Pot wreak alone or with just a few trusted deputies? Obviously, a ruler needs many others to cooperate with him. Importantly, cooperation does not require understanding. Most people will not grasp the tyrant’s project, much less endorse it. But one way or another they will go along actively and passively (abstaining from resistance). Why? Because they have learned — from parents, teachers, youth-group leaders, peers — that that is just what one does: one respects the “law of the land,” which is seen as the will of the legitimate ruler(s) or of the people, depending on the reigning ideology, but is in fact merely a series of decrees (statutes, executive orders, etc.) widely believed to be authoritative. So coercion is unnecessary. But even when it is required, the tyrant still needs the cooperation of those called on to enforce the decrees. Should they defy their orders, the cooperation of others will be relied on to have those orders carried out. Etc. The ruled always outnumber the rulers. If no one cooperates, the tyrant must abandon his project or do his own dirty work. But that means the damage he could inflict would necessarily be small-scale. Moreover, acting alone, the tyrant would be exposed as a fraud, a mere freelance thug. In the end, evil systems can exist only because enough unthoughtful people cooperate with authority, unthoughtful in Hannah Arendt’s sense, that is, in the sense that they never ask themselves if they have a better reason to respect the authority’s decrees than “it’s the law.” I think this is what Arendt meant by “the banality of evil.”
(For more, see my “Come and See the Anarchy Inherent in the System!” and the chapter “The Constitution of Anarchy” in my book America’s Counter-Revolution: The Constitution Revisited.)