Is the novel ultimately optimistic or pessimistic? What is the evidence?
Since one of the major themes of the book is that man is inherently evil and civilization is what keeps that evil somewhat in check, I'd say it's pessimistic. Golding, having been greatly affected by his service in WW II, felt that people had an inner savagery that came out in certain circumstances, such as war, where the savage nature was allowed to emerge. He believed that if the confines of society were removed, as with the boys on the island, the inner beast would come out. The boys in the story crash land on the island because of a war going on in their society. They have the chance to create a new, peaceful, productive society free from any of the adversarial elements of their former society. Instead, they create an increasingly more chaotic existence in which all the negatives are highlighted - ignoring the need for shelter in order to have fun - offering up sacrifices to unseen and non-existent beasts - killing one another. The ultimate evidence of this pessimism is in the last chapter. Ralph has to run and hide like a wild animal being hunted and he has to become savage in order to survive. Finally, when he is rescued, he cries for the deaths of Piggy and Simon, for the realization that he is as savage as the others, and for the realization that all men have evil in their hearts. That's a pretty depressing ending.