In "Lord of the Flies" at the end of Chapter 9 there's a lot of imagery. Why does he use it and how does it inform the theme?

The great detail and imagery serves to put the scene in slow motion.  Just as movies use slow motion to emphasize important moments, Golding magnifyies the details of the scene.  This is an incredibly significant moment in the book-Simon is killed.  It is where the boys truly give in to their more barbaric natures.  The way Golding describes the boys adds to his theme of the barbarism that lies in each of our hearts. He calls the boys "strange, moonbeam-bodied creatures with fiery eyes...attendant creatures...trailing vapors", which emphasizes the fiery violence that had taken hold of them, turning them from humans into "creatures".

The landscape descriptions focus on nature's consistent, calm and predictable course, logical and steady, which is a good contrast to the crazed action that has just occurred.  Golding describes "steadfast constellations" a "solid core" turning, "sun and moon pulling", "the great wave of tide flowed".  All of these calm, predictable elements of nature contrast the chaos of the previous scene.  Also, it as almost as if Simon has become a sacrifice for nature to accept; nature is at times barbaric, and sacrifices are made to its wildness.  After Simon's death, nature accepts him, and makes him beautiful as "the line of his cheek silvered" and "the water rose farther and dressed Simon's coarse hair with brightness."