The ending rescue scene is all-important for the author to make the final profound point that barbarism and violence are not only possible in times of war or in evil people, but that it is contained within each of us, if we give it free reign. He sets the rescue during the most intense part of the novel, where Jack and his boys are hunting Ralph down, deliberately trying to kill him. Before this, the murders of Simon and Piggy were either accidental, or rash actions gone bad. But now, Jack and his clan have let their more animalistic, violent nature completely take over, and the hunt is on. As the soldier runs into Ralph on the beach, his questions reflect most of society's attitude about children, that they are only capable of play. He asks, "What have you been doing? Having a war or something?" His question is unintentionally glib; kids play war, cops and robbers, armies and soldiers all of the time, and his question is referring to that, and it is a gross understatement of the real war that was going on. This emphasizes Golding's point that we are all capable of evil, even small children, if left without guidance, rules, civility, morals, and enforcement of all of them.
The ending is the most important part of the book, and doesn't make it seem less realistic; it intensifies the theme, makes the reader think even harder about what has occurred, and really hits home as the contrast between civility and barbarism are brought right up against each other to view.