A proper treatment of the Socratic claim that no one knowingly does evil could be a very long discussion; we will only touch on it briefly. First, to clarify, Socrates does not mean to suggest that no one ever commits an evil act out of hatred or selfishness. Rather, he wishes to suggest that hatred, selfishness, and any other source of evil action can ultimately be traced back to ignorance. For Socrates, hatred between people is the result of misunderstanding or miscommunication, and selfishness is the result of deficient self-knowledge. If we knew ourselves and others fully, and had a full understanding of the facts at hand, we would never commit an evil act.
Another question regarding the Socratic claim is how far we can take it to be a claim at all. Interestingly, Socrates never seems to directly assert this claim, and he certainly never argues for it. Usually, he just implicitly treats it as though it were self-evident. If it is not a positive claim that can be argued for, Socrates is not violating his claim that he has no positive knowledge in any specialized field. But if it is not a positive claim, what sort of claim is it? Perhaps we could argue that it is an intuition, one that has no evidential basis. Alternatively, we could read it as an empty claim, asserting nothing more than the fact that if we knew everything, we would know what was best for ourselves. There is no easy answer to this question that presents itself to us in the text.
For a man such as Socrates, who claims to be so committed to improving the state and its citizenry, it would strike the jury as odd that he places so little emphasis on public affairs. This, he explains, is the advice of his supernatural sign, the voice in his head that warns him against such activities. This sign could be taken to be one of the supernatural things that Socrates believes in, which he used as evidence of his belief in gods during his earlier cross-examination of Meletus. Interestingly, though Socrates is persists in saying that this voice is of supernatural origin, he nowhere suggests that it has to be a god, in spite of his earlier assumption that all supernatural things are either gods or children of gods.
In any case, the "sign" is right: Socrates would not last long in public affairs. His present trial is just one of many cases in Athenian history where justice was unfairly suspended when the safety of the state was thought to be at stake. Socrates' emphasis is on the ethical life as expressed on the personal level and through self-knowledge. In Socrates' view, the health and prosperity of the state would follow if every one of the citizens were wise and virtuous, but no set of laws can ensure such health and prosperity if the citizens act unjustly. These considerations were especially pertinent following the Peloponnesian War, as Athens fell into decline.