Question

I  don't have any answers here or anything witty to say here, this was just something that I wondered while reading. 

In Crito, Socrates asks Crito why they should consider the opinion of the many, because the only people with considering were the good men and they would remember it the way it happened.  Crito argues that the many can do the "greatest evil to anyone who has lost their good opinion.  Socrates responds by saying that he wished they could to the greatest evil, because then they would have the ability to do the greatest good. "But the truth is, that they can do neither good nor evil: they cannot make a man wise or make him foolish; and whatever they do is the result of chance."  

I don't really understand why Socrates insists that the people cannot do good nor evil.  He states that some of the people are good, so are they simply good in their nature without the ability to actually do good? Or is he only talking about the people who are not good? And if this is the case, does this mean that the people who are not good are not good because they don't have the ability to be good?