Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 551.
“He who, when he may, forbids not sin, commands it.”
Troades (The Trojan Women), line 291 (Agamemnon)
Alternate translation: He who does not prevent a crime, when he can, encourages it. (translator unknown).
Tragedies
Original
Qui non vetat peccare cum possit, iubet.
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Seneca the Younger 225
Roman Stoic philosopher, statesman, and dramatist -4–65 BCRelated quotes

“He who has never sinned is less reliable than he who has only sinned once.”
Source: Antifragile: Things That Gain from Disorder (2012)

II – The General and His Troops.
"Generals and Generalship" (1939)

“The guilty one is not he who commits the sin, but he who causes the darkness.”
Source: Les Misérables

Source: In Job's Balances: on the sources of the eternal truths, On The Philosophy of History p. 247

“He who has not sinned lacks a stone.”
Ron English's Fauxlosophy: Volume 2 (2022)

“He who does not punish evil commands it to be done.”
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.