If Socrates had an enemy it would be the sophists, and the greatest sophist was Protagoras. The sophists traveled throughout the Greek world teaching wisdom, or really rhetoric; the ability to make a weak argument look stronger and to trap people in their words. Protagoras was the most famous of these and is known for his quote, "Man is the measure of all things". This is a sort of moral relativism; if we feel something is good, just, etc., than it is, even if someone else feels differently based on their own perspective. For example, what is good for the undertaker is not good for the person who just died. The truth is somewhere in the middle.
Obviously some people (like the Catholic church) would disagree strongly with Protagoras. I recognize that what seems moral and acceptable in one culture or time is not acceptable in another place or time. I think that is part of being open-minded, well traveled and understanding.
This dialague concerns a discussion between Protagoras and Socrates to determine if virtue can be taught. Socrates says that "virtue" is knowledge and that "evil" is another name for ignorance. It is impossible to be "bad" if we know "good" (i.e. have knowledge). Socrates believes that no one willing commits an evil act (steal, lie, etc), unless they slip into a state of ignorance and become unaware of how their actions will adversely affect them.
I have my own theories on why people commit evil acts or harm themselves. It is a constant struggle for some people, including myself, to overcome their appetites. I do not think it is a lack of knowledge, but a lack of good judgement, which is an emotional response not a logical one. I will "harm" myself because it feels good at the time, just like a jury can be swayed by arguments that appeal to their emotion despite the evidence on the contrary.
Plato does parse words, which I think is good and bad. Of course we want to define what it is to be just, good, holy and virtuous, but he also makes arguments pivot on the definition of “being” compared to “becoming” and to “will” instead of to “wish” as he does in Protagoras. It does make me feel as though ancient Greek and English are not so different; but semantics are the same in any language, modern or ancient. Socrates is debating if “hard” is the same thing as “evil” from an ancient poet, Simonides. Likewise, the fact that a sentence ends in “truly” apparently can change the whole meaning of a phrase. I am impressed that language could be used like a tool, almost like math to calculate the meaning of a message (which I guess would be called “Logic”). However, sometimes words lack clear definitions (unlike numbers!). I believe this is where this where Plato gets into trouble.