From Plato's Meno. Socrates explains why no one ever knowingly does evil.
http://classics.mit.edu/Plato/meno.html
1. What is "evil"?
2. How does Socrates show that no one desires evil?
3. Why does Meno initially believe that some people knowingly do evil?
Soc. Then are there some who desire the evil and others who desire the good? Do not all men, my dear sir, desire good?
Men. I think not.
Soc. There are some who desire evil?
Men. Yes.
Soc. Do you mean that they think the evils which they desire, to be good; or do they know that they are evil and yet desire them?
Men. Both, I think.
Soc. And do you really imagine, Meno, that a man knows evils to be evils and desires them notwithstanding?
Men. Certainly I do.
Soc. And desire is of possession?
Men. Yes, of possession.
Soc. And does he think that the evils will do good to him who possesses them, or does he know that they will do him harm?
Men. There are some who think that the evils will do them good, and others who know that they will do them harm.
Soc. And, in your opinion, do those who think that they will do them good know that they are evils?
Men. Certainly not.
Soc. Is it not obvious that those who are ignorant of their nature do not desire them; but they desire what they suppose to be goods although they are really evils; and if they are mistaken and suppose the evils to be good they really desire goods?
Men. Yes, in that case.
Soc. Well, and do those who, as you say, desire evils, and think that evils are hurtful to the possessor of them, know that they will be hurt by them?
Men. They must know it.
Soc. And must they not suppose that those who are hurt are miserable in proportion to the hurt which is inflicted upon them?
Men. How can it be otherwise?
Soc. But are not the miserable ill-fated?
Men. Yes, indeed.
Soc. And does any one desire to be miserable and ill-fated?
Men. I should say not, Socrates.
Soc. But if there is no one who desires to be miserable, there is no one, Meno, who desires evil; for what is misery but the desire and possession of evil?
Men. That appears to be the truth, Socrates, and I admit that nobody desires evil.
