Why does Marcus Aurelius say wrongdoers act involuntarily?

Marcus does not mean wrongdoers are blameless or that their acts are accidents. He means that every soul is "unwillingly deprived of the truth" (Meditations 11.18), so anyone who does wrong is doing it while missing something they would need to see clearly in order to act well. The wrong follows from a mistaken judgment, and no one chooses a mistaken judgment as mistaken. That is the sense in which the act is involuntary.

This is a direct application of the Stoic theory of action. For the Stoics, wrongdoing is never a straightforward choice of evil for its own sake. It is a failure of understanding about what is actually good and bad. Marcus works through this in a short chain of reasoning in 11.18. He starts from the premise that nature orders all things toward one another, so human beings are naturally fitted for cooperation, not conflict. Then he narrows in on the wrongdoer specifically: if people did right, you would have no complaint, and if they do wrong, "it is plain that they do so involuntarily and in ignorance" (Meditations 11.18). The reasoning behind this is that the soul is unwillingly deprived of the truth, and just as unwillingly deprived of the ability to treat each person as they deserve. Notice the evidence he offers for this: people get upset when you call them unjust or ungrateful. If wrongdoing were something people did knowingly and gladly, being named for it would not sting. The sting shows that somewhere they still value being just, even while failing at it.

The common misreading treats this as an excuse: if the person could not help it, then nothing is really their fault, and you should just let it go. That is not what Marcus is doing. He is not suspending judgment about right and wrong. He still calls the act wrong. What he is doing is diagnosing the cause of the wrong, so that your reaction to it can be accurate instead of inflamed. Anger at a wrongdoer usually assumes the person saw the good clearly and rejected it out of malice. Marcus is telling you that this picture is almost never true. The person acted from a false judgment about what was good for them, the same kind of mistake you yourself are capable of making. He makes this personal in the same passage: you too do many things wrong, and even where you abstain from a fault, it may be from cowardice or concern for reputation rather than real understanding, so you are hardly in a position to feel superior (Meditations 11.18).

He also adds a check on your own certainty. You do not even always know whether someone is doing wrong, because acts depend on circumstances you may not see (Meditations 11.18). This does not soften the ethical claim that only virtue is good and only vice is bad. It sharpens where to aim your effort. If wrongdoing is ignorance, then the wrongdoer needs correction or teaching, not your outrage, and your own anger at them is itself a judgment you have made, one you can examine and let go of.

What changes if you take this seriously is not your standards. You still call a wrong act wrong. What changes is your posture toward the person doing it. Instead of treating them as an enemy who has judged clearly and chosen badly, you treat them as someone who, like you, is working with a faculty of choice that can still go wrong. The concrete practice is simple: the next time someone wrongs you, before reacting, ask what they must believe is good for them to act this way. That question does not excuse the act. It replaces contempt with the kind of clarity Marcus was after.