The sad truth of the matter is that most evil is done by people who never made up their minds to be or do either evil or good.

Hannah Arendt (1906-1975) German-American philosopher, political theorist
The Life of the Mind, Vol. 1 “Thinking,” Part 3, ch. 18 (1977)
    (Source)

Sometimes rendered (possibly from the original lecture): "The sad truth is that most evil is done by people who never make up their minds to be good or evil."