In lord of the flies why did Golding choose Simon to tell them that the beast is not a creature but the evil that lies inside of them?

Simon is Golding's Christ-figure in the book. This means that he carries the traits of Christ: he tries to save the boys from themselves. While Piggy has knowledge, Simon has wisdom. He is able to see beyond the fear of the beast that the boys have and understand the truth of their anxieties. It is the boys' own darkness that casts an evil shadow over the island. It is their own darkness unchecked by the laws of civilization that manifests the evil on the island. The boys are not on the same spiritual level as Simon and most of what he says is not understood.