In Lord of the Flies, what is Golding trying to show by having the novel begin in friendship and end in violence?
You are referring to the theme of the book in asking what Golding is trying to bring out. Golding maintained that people were inherently evil and that only the rules of society taught to us as we live in society keep that evilness inside. He felt that mankind had this basic flaw of an inner beast and when stripped of civilization, the inner beast would emerge. In his story, the boys crash land on the island escaping an example of that flaw because the world is at war - people being savage toward one another. If there were no inner beast, the boys would have had an idyllic existence on the island living in peace with each other. The evil inside emerged though and it didn't take long for it to emerge. Jack wanted power, just like many countries who start wars want power. Because of his pursuit of power, Jack became savage quickly. Roger enjoyed the thrill of exerting his strength because he liked watching people cower before him and fear him. His beast didn't take long to come out, either. Many of the other boys were merely followers and their evilness comes out more slowly. Even Ralph eventually let out his evil beast when he grabbed the stick sharpened at both ends that had held the pig's head. He was going to kill or be killed. The only two main characters who never let out their true inner beast were Piggy and Simon - the two who the other boys killed. Golding shows how, stripped of civilization, the boys let the evil inside of each of them come out.