Socrates is written to have said, "There is only one good, knowledge, and one evil, ignorance." Generally, he was saying that people do not do evil, except by ignorance. This statement sounds very simple, and many people think they understand it when they read it; but in my view, they don't.
It seemed to me Socrates was saying that even the worst, most treacherous villains wouldn't be so if they were only more knowledgeable of their deeds. But surely, not all people are simply 'doing the best they can', and are simply misguided or mislead. Many people do indeed confuse right and wrong, but some people know an act is wrong even as they are doing it. As proof of this, they may even feel guilty or ashamed as they do it.
Therefore, when I first read this statement of Socrates, I thought it terribly naive. I was wrong. I wasn't wrong about people; some people do knowingly have malicious intent. But I was wrong about what Socrates was saying. I think many people today are wrong about this, and proceed as though there is no such thing as evil people; as if we're all just 'doing the best we can'. Because they share my once-shallow interpretation of Socrates' words, they mistakenly think he backs up this naive view.
The truth in what Socrates is saying here is not so important because of what it says about people, however. Rather, what he says is important because of the profound thing it says about goodness and virtue.
People who do evil are ignorant. But the common mistake in interpreting this notion is to misunderstand what it is they are ignorant of. Evil-doers with intentionally malicious aims are not ignorant of what is good and what is evil - on this, they are clear, even if they may be unwilling to admit it to themselves.
Instead, what these sorts of evil-doers are ignorant of is the fact that virtue and wisdom are one and the same. They look at virtue as a sort of external set of rules applied on top of life, sometimes limiting our choices and not allowing us to do what is necessary or beneficial to us. These sorts of people tend to be good because of social pressures, or seeking rewards, or fear of punishments, or emotional urges. They will, when they think it to their advantage, do what they know to be evil.
The mistake they make is in their perception of what virtue is. What they don't understand is that virtue is always the wise course of action, and the wise course of action is always the virtuous one. There is never a time when the practical, pragmatic, necessary, efficient, or beneficial thing to do is non-virtuous. If they think so, it is because they're not considering all of the relevant variables in the long term. Their definition of "practical", "efficient", or "beneficial" may be short-sighted.
In summary, intentional evil-doers are ignorant of the fact that virtue and wisdom are synonymous. They are also ignorant of the fact that virtue is both necessary and sufficient for living a truly happy life.
This may be a difficult truth to grasp at first, and may even seem counter-intuitive to some. It's a certain conceptual understanding of how events flow together and how lives and true happiness are affected by different things. Indeed, finally 'getting it' on a deep level is a lot like that moment when your eyes settle into the right configuration to see the 3D picture in a stereogram. But as long as this form of ignorance remains, the person will be retarded in their ethical development to the level of child, and their well-being will suffer as a result.
 Diogenes Laertius, Lives of the Philosophers