Ralph didn't directly kill Simon, but he didn't stop Jack from doing it.
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He is guilty because he didn't stop jack from killing Simon. (Poor Simon best character though)
He feels guilty about murdering Simon; him and the rest of the boys kill Simon at the end of chapter 9
The death of Piggy and Simon
Ralph is as guilty as any of the other boys because he took part in the dance and the subsequent brutal killing of Simon. As he explained to Piggy in the following chapter, "That was murder." He also added "You never really came in. Didn't you see what we-- what they did?"
Ralph and Simon were attempting to build a third shelter on the beach. Ralph was standing outside it placing branches and leaves onto it, while Simon was inside trying to arrange the branches.
he predicts that SImon will reach or he shall return home safely. Simon says, "You'll get back all right." (Chapter 7) Ralph's reaction is unsure and uncomfortable.