During the meeting in chapter 5, Ralph opens up the discussion to the boys' fears of a beast. To Ralph's surprise, Simon takes the conch and says, "Maybe there is a beast." Simon isn't comfortable speaking in public, but he manages to get out the words, "maybe it's only us." After this, Golding writes the line quoted above. Then, seeking to explain, Simon asks the boys, "What's the dirtiest thing there is?" Jack shouts out a vulgar word, probably a word for excrement. This causes the boys to laugh and scream with delight, and Simon's message is lost.
What Simon understands that the others don't is that the thing the boys have to fear most, which he symbolically calls "a beast," is their own inner depravity. When he speaks of something dirty, Jack takes it into the physical realm, but Simon is operating in the abstract world and thinking of moral impurity. The "dirtiest thing there is" is the impulse in every human to hurt others, to destroy nonliving and living things, and to act for self-gratification without regard for consequences. These impulses are already evident within the boys. A child has been killed by the boys' unthinking excitement over making a fire. Piggy's glasses have been broken by Jack in a vicious assault that all the boys laughed at. And now they have missed a chance of being rescued because the boys were focused on killing a pig. Simon alone has the insight to recognize these signs of "mankind's essential illness." That illness, according to Golding, is an innate moral depravity that will cause destruction and death if it is not kept in check.
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.
mankind's essential illness is the necessity to kill