"Simon became inarticulate in his effort to express mankind's essential illness" What does this mean? What is the "illness"?(Ch.5,p.89)

Man's "essential illness" as presented by Golding, is man's inherent evil nature. This illness (man's tendency towards evil) is represented by the boys decent into violence and depravity - ultimately the death of Simon himself. Simon was the most intuitive of the group and saw that this starting to become a problem in the group. He just was not able to articulate - or find the words to explain - this to the rest of the boys. Even if he could, most likely the group would not have understood or cared about what Simon was saying.