Lord of the Flies should not be considered a rescue, or adventure story, but rather a statement on society. The entire novel revolved around removing individuals from the rules and expectations of civilized society and seeing the effect that their new freedom would have. Jack spiraled down into savagery, bringing most of the other boys with him. Evidence of his savage tendencies was his embrace of his long hair and dirty clothes. Instead of being repulsed by blood, he finds it funny and even smears it on his face. Ralph, however, rejects his long hair and dirty clothes and longs to return to civilization. The rescue fire is a priority for him and not Jack because Jack enjoys the freedom to act in ways that would be unacceptable in his former civilized society.
The boys' rescue happens right as Jack's savagery is coming to a peak and he is about to kill Ralph in cold blood. The reader, at this point, is judging Jack to be the most savage person; he is exhibiting behavior that is completely unacceptable to the reader. We judge him and feel great relief that Ralph survives and the boys are being brought back to civilization.
However Golding does not end the novel with Ralph's emotional release of all the evil that occurred on the island. Instead, he adds the following two sentences to conclude the novel:
The officer, surrounded by these noises, was moved and a little embarrassed. He turned away to give them time to pull themselves together; and waited, allowing his eyes to rest on the trim cruiser in the distance.
By ending the novel with the image of the cruiser, we are reminded that the boys are being rescued, but are being brought back to a civilized society that is in the midst of a war just like the one from which they are being rescued. Ending the novel with Ralph being killed would present the idea that savagery overtly wins in the end. By ending the novel with rescue, the idea presented is that savagery is a subtle force that is ever-present, even in civilized society.