Why do people do wrong, according to the Stoics?

Epictetus builds his answer from a claim about the mind itself. Assent, the act of taking something to be true, only ever follows the appearance of truth: "the fact that it appears to be true" is the whole cause of assenting to it (Discourses I 28). You cannot make yourself believe it is night when it plainly looks like day. The mind is built to follow what seems true and to move toward what seems good, and it cannot do otherwise. From this Epictetus draws the conclusion that matters: nobody assents to something believing it false, and nobody pursues something believing it bad for them. "Every soul is unwillingly deprived of the truth," he says, borrowing the line from Plato (Discourses I 28). Wrongdoing, on this view, is never a clear-eyed choice of evil over good. It is a mistake about which is which.

Discourses I 18 applies this directly to people we are tempted to call vicious. Epictetus imagines someone objecting: but these men are thieves and robbers. His answer is not to excuse the theft but to relocate the problem: "they are mistaken about good and evil" (Discourses I 18). The thief has judged, wrongly, that the object he steals is worth more than his own integrity. That judgment is false, but from inside it, stealing looks advantageous. Epictetus goes further than diagnosis. He asks whether such people should be destroyed for it, and answers that this would be exactly as inhuman as destroying someone for being blind or deaf (Discourses I 18). The wrongdoer has lost the greatest thing a person can lose, a will or faculty of choice functioning as it should, and that loss is itself the harm. His verdict: pity him rather, and drop the readiness to be offended and to hate (Discourses I 18).

The common misreading is to hear this as a kind of moral relaxation, as if the Stoics think wrongdoing does not really matter or should not be resisted. That is not the claim. Epictetus does not say the theft is fine, and he does not say we should stop correcting it. He says: show them their error, and you will see how they desist from it (Discourses I 18). Correction, not punishment fueled by anger, is the fitting response, because anger treats the person as if they saw clearly and chose badly anyway, when in fact they did not see clearly at all.

Marcus Aurelius opens his Meditations with the same premise turned into a daily exercise. He tells himself to expect, each morning, to meet the busybody, the ungrateful, the arrogant, the deceitful, and to remember why: "all these things happen to them by reason of their ignorance of what is good and evil" (Meditations 2.1). Because he has seen what the good really is and what the bad really is, he can neither be harmed by their error nor be angry at people who share his own nature and, on his account, the same portion of intelligence (Meditations 2.1). To act against one another over this is to act against nature, since we are made for cooperation the way hands, feet, and eyelids are made to work together (Meditations 2.1).

Behind both passages sits the deeper Stoic claim that the faculty of choice is the one thing fully up to a person, and that only its condition, not the state of the world, is genuinely good or bad (Handbook 1, Discourses I 1). The wrongdoer has confused an external, money, status, safety, for something worth more than the health of that faculty. That confusion is the disease. Anger is the wrong medicine because it does not touch the judgment causing the trouble and instead adds a second false judgment of your own.

What this changes in practice is where you put your effort. Instead of rehearsing how someone wronged you, ask what false belief about good and evil could make that action look sensible to them. That question does not excuse the act. It tells you what would actually need to change, and it keeps your own faculty of choice, the only thing truly yours, out of the wreckage.