The sun allows all visible things to flourish without becoming part of it. What does this statement mean? According to Plato the real nature of the existence is Eidos, viz. Idea while the material world is the latest and poorest expression of being. The universe is plenty of ideas, each of one is unique, eternal, unalterable and complete. All of them are made of spirit. As a result we can't accept the possibility of evil.
The concept of evil means not only a bad and violent behaviour aiming to destroy life in general but also corruption, decay and finally death. This general understanding of evil contradicts the platonic universe and therefore Platon rejects evil as an idea within the world of ideas. First, ideas are eternal. Assuming that evil exists means also that ideas could decay. Secondly the meaning of evil could imply that ideas participate in nature which is inconsistent with his theory of archetypes. According to Plato although nature can participate in the world of ideas, these are completely independent of nature because in the end all living beings and physical things are just a copy, a reflexion of the reality made of ideas. To me, reality is decided in degrees and the physical world as humans know, would be the lowest level of reality.
Does evil exist at all? According to Plato, evil has no reality. What we understand as evil, ist just lack of good, the poorest and lowest level of reality. Such a thing would be evil. Therefore the idea of good is key to understand the link between knowledge and moral, but especially, to understand why human beings behave badly. Nowadays this relationship seems disconnected. Knowledge ist related to objects, numbers and Physics while the idea of good is part of moral and social studies. To Platon is the contemplation of ideas, especially the idea of good fundamental not only to know well but also to behave properly. Without this idea of good humans beings can be manipulated, misleaded and prompted to do wrong.