“A slave has no morality, because he cannot choose between good and evil. He has only a derivative morality—that of his masters.”

Variant: No man chooses evil because it is evil; he only mistakes it for happiness, the good he seeks.

A Vindication of the Rights of Men (1790)

Source: Self-Help; with Illustrations of Character and Conduct (1859), Ch. I : Self-Help — National and Individual

Source: Cannibals All!, or Slaves Without Masters (1857), pp. 125-126

Source: An Essay on Aristocratic Radicalism (1889), pp. 30-31

“Welt muss mehr denn je diese Botschaft hören,” Giessener Allgemeine Zeitung, Giessen, Germany, April 12, 2005.
Attributed

Source: No Treason (1867–1870), No. VI: The Constitution of No Authority, p. 24; the first sentence here is widely paraphrased as: A man is no less a slave because he is allowed to choose a new master once in a term of years.
Context: A man is none the less a slave because he is allowed to choose a new master once in a term of years. Neither are a people any the less slaves because permitted periodically to choose new masters. What makes them slaves is the fact that they now are, and are always hereafter to be, in the hands of men whose power over them is, and always is to be, absolute and irresponsible.
The right of absolute and irresponsible dominion is the right of property, and the right of property is the right of absolute, irresponsible dominion. The two are identical; the one necessarily implying the other. Neither can exist without the other. If, therefore, Congress have that absolute and irresponsible lawmaking power, which the Constitution — according to their interpretation of it — gives them, it can only be because they own us as property. If they own us as property, they are our masters, and their will is our law. If they do not own us as property, they are not our masters, and their will, as such, is of no authority over us.
But these men who claim and exercise this absolute and irresponsible dominion over us, dare not be consistent, and claim either to be our masters, or to own us as property. They say they are only our servants, agents, attorneys, and representatives. But this declaration involves an absurdity, a contradiction. No man can be my servant, agent, attorney, or representative, and be, at the same time, uncontrollable by me, and irresponsible to me for his acts. It is of no importance that I appointed him, and put all power in his hands. If I made him uncontrollable by me, and irresponsible to me, he is no longer my servant, agent, attorney, or representative. If I gave him absolute, irresponsible power over my property, I gave him the property. If I gave him absolute, irresponsible power over myself, I made him my master, and gave myself to him as a slave. And it is of no importance whether I called him master or servant, agent or owner. The only question is, what power did I put into his hands? Was it an absolute and irresponsible one? or a limited and responsible one?

The Groundings with my Brothers https://archive.org/details/TheGroundingsWithMyBrothers (1969)
Context: We were told that violence in itself is evil, and that, whatever the cause, it is unjustified morally. By what standard of morality can the violence used by a slave to break his chains be considered the same as the violence of a slave master? By what standards can we equate the violence of blacks who have been oppressed, suppressed, depressed and repressed for four centuries with the violence of white fascists. Violence aimed at the recovery of human dignity and at equality cannot be judged by the same yardstick as violence aimed at maintenance of discrimination and oppression.

Variant: The important thing is moral choice. Evil has to exist along with good, in order that moral choice may operate. Life is sustained by the grinding opposition of moral entities.
Source: A Clockwork Orange