The idea of evil is much more simple than most people believe. It is doing something morally wrong. Evil doesn’t need to be murder, evil can be as simple as lying about where you were last night. If you know its wrong, and you do it anyway, that evil. The boys in the book don’t understand this idea so well. As near the end, they begin to leave sacrifice for the beast. What they don't understand is that, you can’t cure evil. It's not a disease. It's a part of life. The boys think that evil doesn't exist on the island. That they are all perfectly clean. Yet, they don't recognize what they do as evil. Therefore they believe it's not there. But what they don't realize is that the beast is the evil that none of them will admit too.
Simon, the only one of the boys who reaches the realization that beast is the evil within each one of them, figures it out. He figures out the secret of the beast and he understands it on a level that no one else on the island does. This directly relates to Abraham Maslow’s hierarchy of needs. Simon is the only one of the boys who successfully makes it to stage five, Self Actualization. He comes to realize that the