The boys are alone on the island because their plane crashed. They were a group of boys from a British Prep. School during a war and their plane was shot down. They were being evacuated for their safety and crashed in the process of the evacuation. The pilot – the only adult mentioned as accompanying them on the plane -- died in the plane crash. There is evidence that there have been other people on the island, for example the parachutist, but at this point when the story is taking place the boys are all alone to fend for themselves.
William Golding's decision to have a group of British boys, ages 6 to 12, crash land onto an uninhabited island is significant to the story's allegorical message. Golding suggests that man's inherent wickedness is ultimately responsible for the destruction of civilization and our environment. Golding, a participant in WWII, witnessed human depravity firsthand, and with the advent of the atomic bomb, knew the extent of destruction that human beings were capable of committing.
Throughout the novel, the boys have an opportunity to develop a civil society void of adult influence and previously established institutions. As the novel progresses, the boys begin to descend into savagery and follow their insatiable primitive instincts and desires. By the end of the novel, three boys have lost their lives, and the island is destroyed by an uncontrollable fire. The plight of the stranded group of boys symbolically represents Christian theology's concept of "the fall of man" when Adam and Eve succumb to temptation and are kicked out of the Garden of Eden. The island metaphorically represents the Garden of Eden, and the boys' decision to become savages correlates with Adam and Eve's decision to eat the forbidden fruit. If there were previously established institutions on the island with an already existing population, Golding's allegory would differ. William Golding was attempting to portray how man's inherent wickedness would ultimately result in destruction, which is why he chose the setting to be an uninhabited island. The boys are essentially left to their own devices to create a civil society rooted in morality and humanity, yet cannot successfully maintain one.