Plato's Theory of Evil
Plato's theory of evil states that evil comes from people that want something and fail at getting that desire might give it a second try and do whatever they think it might take to get that desire, even if it is morally or legally wrong. They may steal or kill anyone that stands in the way of obtaining what they want.
In his opinion people act nefariously because they experience frustration when they do not obtain what they desire.
In order to prevent this, we must cultivate the intellect & spirit and don't give in to all desires. He says that God is perfectly good and evil is alien to his nature. He says to think that he is responsible for any evil is contradictive.
- The Republic, Book IV (375 BC)