How is the end appropriate to the narrative and themes of The Lord of the Flies?

The concluding episode is appropriate for the narrative and themes of The Lord of the Flies because Jack and the hunters have completely descended into innate savagery and its destructive nature. And, although there is a rescue by civilization, the presence of the naval officer and his warship do not assure readers that goodness and order will be completely restored. 

Of course, there is no more destructive force than fire. In the final chapter, with the conflagration set on the island by the hunters in their savage pursuit of Ralph, the symbolism of this destructive act underscores Golding's contention that man is far from inherently good. This is a theme which Golding has created as a counterpoint to Robert Michael Ballantyne's The Coral Island: A Tale of the Pacific Ocean, a novel in which English schoolboys retain their Christian values and British civility until their escape from pirates and cannibals.

In the final chapter as Ralph hides in a covert listening for the approach of the hunters, he sees Bill. 

But really...this was not Bill. This was a savage whose image refused to blend with that ancient picture of a boy in shorts and shirt.

Still, Ralph wants to believe that the boys are not evil, and that Simon's death was an accident. But, then, he encounters the Lord of the Flies, and, as he kneels in the shadows, he senses his terrible isolation and acknowledges, "They were savages it was true....he knew he was an outcast." Not long after these thoughts, Ralph finds himself smoked out from his hiding spot; however, in doing so, the hunters have also set the entire island on fire. Thus, they have destroyed what the boys at first perceived as a paradise. Clearly, this act demonstrates their evil natures.

Wondering how he can save himself, Ralph sees a shelter burst into flames. Then, as this fire flares, he spots "the glitter of water." Ralph runs until he falls down; then. he staggers to his feet, expecting "more terrors." Instead, he finds himself looking up at a white-topped cap with an anchor and gold foliage. A naval officer standing on the beach looks down at the boy in astonishment. 

"We saw your smoke. What have you been doing? Having a war or something?"

Further, he asks Ralph if anyone has been hurt or killed and how many of the boys there are. When Ralph tells him that there have been two killed and he does not know how many boys there are, the officer disapprovingly remarks,

"I should have thought that a pack of British boys...would have been able to put up a better show than that--I mean--....Like the Coral Island."

Ralph "gives himself up" to the officer, weeping for

...the end of innocence, the darkness of man's heart, and the fall...of the true, wise, friend called Piggy."

Furthermore, while civilization saves Ralph and the other boys from savagery, the officer looks out at the warship waiting in the distance. This act appropriately reminds readers of Golding's theme of the inherent evil in man that has been depicted by the murders of Simon and Piggy and the fiery pursuit of Ralph in the final chapter.