Where all are guilty, no one is; confessions of collective guilt are the best possible safeguard against the discovery of culprits, and the very magnitude of the crime the best excuse for doing nothing.

Hannah Arendt (1906-1975) German-American philosopher, political theorist

Essay (1969-02-27), “Reflections on Violence,” The New York Review of Books
    (Source)

Revised and collected in Crises of the Republic, "On Violence" (1972).