What is "mankind's essential illness"?

What Golding alludes to as “mankind’s essential illness” he doesn’t identify, leaving it to the reader to determine the nature of the darkness Simon recognizes in human beings. Literary critics frequently conclude that it is the presence of evil and that the novel expresses Golding’s view that man is evil by nature. How then can evil be defined in the context of the narrative? As a spiritual concept, the antithesis of goodness? As the chaos created by the savage, unrestrained Id of the human psyche? Specific passages in the novel support both interpretations. However, a scene in chapter 4 suggests that “mankind’s essential illness” can be defined another way, as well, based on the behavior of a littlun playing on the beach.
At the ocean’s edge, little Henry digs runnels in the sand with a stick and watches them fill with water; as the tide recedes, tiny sea creatures washed ashore are trapped in the shallow channels he has created. Fascinated with the “tiny transparencies,” he pokes the sand with his stick, directing them into the runnels where he wants them to go. Golding describes Henry’s behavior in a passage that implies the profound significance of his actions:
He became absorbed beyond mere happiness as he felt himself exercising control over living things. He talked to them, urging them, ordering them. Driven back by the tide, his footprints became bays in which they were trapped and gave him the illusion of mastery.
What drives Henry’s behavior is then underscored through literary parallelism. As Henry controls the creatures trapped at his feet, Roger controls him, surreptitiously throwing stones so that they land near Henry, prompting him to determine their source and confusing him when he can’t. Through this scene, another interpretation of “mankind’s essential illness” is suggested by the littlun’s actions, and it is validated by Roger’s. The darkness in human nature, their behavior implies, is man’s innate drive to dominate that which exists separate from the self. Moving in ever expanding circles, like the “spreading rings” in the water created by Roger’s throwing stones, it is the blind expression of human will that plunges the island into anarchy and the world beyond the island into war.