Hannah Arendt found loneliness at the root of totalitarianism
The last pages of The Origins of Totalitarianism contain a claim that still startles: the precondition for mass terror wasn't hatred or fanaticism — it was a very particular kind of isolation.
Arendt distinguished between two states that look similar from the outside. Solitude is productive: you're alone, but in conversation with yourself, capable of judgment. Loneliness is something worse — the experience of having no reliable world at all, of being abandoned by others and eventually by one's own sense of self.
Lonely people, in Arendt's analysis, become uniquely susceptible to ideological systems that promise total consistency. The ideology explains everything through a single iron logic — racial destiny, historical inevitability, the hidden hand of conspiracy — and for someone with no stable social reality to push back against the theory, the theory is seductive precisely because it removes the burden of independent thought.
Totalitarian movements didn't require true believers so much as people who had nothing else. The apparatus then organized that loneliness into mass action, replaced individual judgment with collective identity, and made the movement itself the only source of meaning.
Arendt was writing in 1951, drawing on the ruins of Nazi Germany and the early Soviet terror. Her conclusion was not that evil requires monsters. It requires people who have lost the practice of being alone with themselves — and found in its place a movement that does their thinking for them.