Since one will always do what one believes is right.
Socrates held that no one is evil on purpose. If it happens that one is evil is due to ignorance of what is good in each case.
This attitude is common in various philosophies based on the virtue of knowledge, including Indian philosophy.
Thus one even if evil can be made virtuous by acquiring proper knowledge
Socrates claimed that the famous oracle of Delphi appointed him the task to examine the concepts people held as true. Socrates could do that because he was aware of his own ignorance, thus by carefully examining what people thought about certain things and using proper rules of reasoning, both him and the other person would arrive at the truth. Thus the person would acquire true knowledge and thus become virtuous.
This was called the midwife method by Socrates, because like a midwife, it only helped the other person arrive to the truth which is already inside.
This was taken further by Plato, in the sense of the theory of eternal forms which represent eternal truth. Since truth is eternal and unchangeable, there is no learning of novel truths, only remembering of already existing ones..