“He who passively accepts evil is as much involved in it as he who helps to perpetrate it. He who accepts evil without protesting against it is really cooperating with it.”

1950s, Three Ways of Meeting Oppression (1958)
Context: To accept passively an unjust system is to cooperate with that system; thereby the oppressed become as evil as the oppressor. Non-cooperation with evil is as much a moral obligation as is cooperation with good. The oppressed must never allow the conscience of the oppressor to slumber. Religion reminds every man that he is his brother's keeper. To accept injustice or segregation passively is to say to the oppressor that his actions are morally right. It is a way of allowing his conscience to fall asleep. At this moment the oppressed fails to be his brother's keeper. So acquiescence-while often the easier way-is not the moral way. It is the way of the coward.

"A River Runs Through It", p. 22
A River Runs Through It (1976)

Chi non punisce il male comanda che si faccia.
The Notebooks of Leonardo da Vinci (1883), XIX Philosophical Maxims. Morals. Polemics and Speculations.
Variant: He who does not punish evil commands it to be done.

Source: Why Men Love Bitches: From Doormat to Dreamgirl—A Woman's Guide to Holding Her Own in a Relationship

Source: Brisingr

Source: Works and Days

Source: Why Men Love Bitches: From Doormat to Dreamgirl—A Woman's Guide to Holding Her Own in a Relationship

Maxim 459
Sentences, The Moral Sayings of Publius Syrus, a Roman Slave