The boys have been traumatized by the attack. The fact that all the adults on the aircraft had been killed, adds to their anxiety - they are unsafe, vulnerable and insecure. They have just witnessed the destruction of something which was meant to provide them safety and bring them to a safe haven.
The island would provide them safe refuge and stability. It would be a place where they could set up base, organise and plan for rescue. Of course, this would be the logical and common-sense approach - best illustrated by Ralph and Piggy. The plan would be to organize, establish and plan.
The island itself presents no threat to the boys and can be deemed a safe haven. Ironically, the danger lurks within themselves, epitomized by Jack and his desire to hunt as well as Roger - both are presented as inherently evil.
In contrast, Jack and Piggy represent what is good and wholesome, logical and relevant. The island to them provides refuge, a place to settle and create structure, a haven in which they can plan for rescue.
However, Jack and his followers deem the island to be a place of freedom, a haven for pernicious pleasure and delight, free from adult interference. A place where they can indulge their every whim.
The island, in this sense then, becomes the one place where the contrast between anarchy and control, between good and evil, between civilized behavior and savagery is ultimately brought into focus. The final question is then: Who wins?
The attack increases the grimness of the entire situation. The boys' collapse into savagery on the island merely reflects what has already happened to society at large, in adult civilization. As in the world of adult civilization, the boys do at first try to adhere to set of rules, to co-operate, but this facade soon breaks down and all-out war ensues. War is a recurring feature of supposedly modern, advanced, enlightened, mature societies. What happens on the island is a microcosm of the process that afflicts adult civilization.
The book was of course written in a period where the fear of a nuclear holocaust was very real, and the cataclysm of two world wars was still an all-too recent memory. The book, like many others of the time, reflects a deep sense of pessimism about where the whole of civilization was headed, and questioned, indeed, whether civilization could ever be anything more than a veneer for the primitive instincts lurking under the surface, which all too often break out in the form of war. Even decent, rational individuals like Ralph are seen to become engulfed in the chaos - although the presence of such individuals also gives some cause for hope.
There are at least two important reasons for staging this story in the wake of an attack:
- The attack provides an "excuse", so to speak, for the boys to be on the island without a single adult. We would assume that a group of adults, charged with caring for a group of boys, would be virtually inseparable from them. The attack is a means of getting the adults out of the equation, so we can see how the boys will act on their own. The attack itself isn't terribly important.
- The attack affects the initial tone of the story; it provides a sense of anonymous danger, of being hunted, of it not being safe, not on this island or anywhere else. The fact that the attack is not described and the attackers not named makes the attack into a sort of primal force of nature; death has brought the boys to this island, and there is nothing and no one who can protect them from it. This sense of dread manifests in the form of the Beast.