Arendt was struck by Eichmann's utter ordinariness. He was neither a sadist nor a fanatic; he had no particular hatred of Jews and no deep ideological commitment. He was a bureaucrat who had found, in the Nazi machinery of destruction, an opportunity for career advancement. He did not think about what he was doing. He organised the transport of millions to death camps with the same mentality he might have brought to any logistics problem.
The "banality" in Arendt's formula is not a claim that the Holocaust was insignificant — it is a description of the perpetrator's psychology. What made Eichmann evil was not a wicked will but the absence of a thinking ego — the failure to exercise the distinctly human capacity to think from the standpoint of others, to imagine the consequences of one's actions, to engage in the internal dialogue that Arendt identifies as conscience.
The concept of the banality of evil is as much a political as a moral claim. It suggests that catastrophic evil can be produced by systems that make individual judgment unnecessary — that the greatest danger lies not in monstrous individuals but in institutions that dissolve the requirement to think. The lesson for political design is structural: any system that allows ordinary people to commit extraordinary evil by following orders has failed the test of human dignity.
The phrase "banality of evil" appears in the subtitle of Eichmann in Jerusalem: A Report on the Banality of Evil (1963). Arendt's later work "Thinking and Moral Considerations" (1971) develops the link between the absence of thinking and the capacity for evil.