It was Socrates (but probably through Plato's writing). I can't remember the exact source in this case.
Hmm, do you really think most criminals believe themselves to be acting justly? I think they know full well that what they do is "wrong" but since they can get away with it, they figure why not?
At least in the US, I believe a majority of prison inmates get there initially by means of gang violence, and all it's related crimes such as drug dealing and robbery. Most of these people will tell you that it's simply a way of life, particularly true for people on the edge of society with few other means for advancement. So yes, I think most criminals do believe their actions are just.
Actually, now that you mention it, I believe Plato was (consciously or unconsciously, due to his aristocratic background) referring to the unjust in positions of power, maybe not so much criminals. As in, unjust men with long-term planning.
In that case, it is a convincing argument. Consider that most dictators and despots seem to think they are in the right (Ww2, etc.) Compare this to "normal" criminals, who seem to have poor impulse control (lack of long-term planning. )
Of course, that has nothing to do with the article. I've brought up an argument and then dismissed it just as quickly :)
I'm not sure I'd dismiss your argument so quickly. If one considers the "just" to be the best decision, given the circumstances, then most criminals are trying to be just. They may badly need money for their family. Or perhaps to feed a drug habit (In which case, as you said, perhaps they are ignorant of what really is just.) If they didn't think it was the best decision, for some good, why would they do it?
As such, these unjust people should not be punished for their lack of knowledge, but instead educated or rehabilited, as their unjust actions were simply a result of not knowing what the "just" really is.
A modern metaphor for this might replace "knowledge of the just" with "biology that can resist impulse," or something similar. The problem, which was just as valid in Greece as it is now, is the question of "What is just?," or "Is a non-impulsive brain chemistry desirable?".