In Chapter 5 of "Lord of the Flies", what is the dangerous idea that Simon has?

Simon's dangerous idea is that the boys themselves might be the true beast. He realises, but is too inarticulate to express, "mankind's essential illness": the fear and lack of spiritual power which creates an imaginary beast, and eventually, the need for violence, for a savage tribe, and for splitting up the civilisation into violent factions.

Simon has realised what Golding himself calls, in the last paragraphs of the novel, "the darkness of man's heart".