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).