Because they have just noticed what Piggy noticed some time earlier: that Ralph is losing his capacity to think and concentrate on the importance of being rescued. He keeps having 'blanks' when his mind drifts or clouds and he cannot remember what he was saying or why.
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The strange thing that Ralph notices when the boy are playing around with the rolling rocks is smoke. What he is seeing is not real though.
In Chapter 10, Piggy suggests that they let the fire go out at night and relight it every morning because he concludes that the smoke will not be visible at night. At this point, Ralph is frustrated that Jack has started his own tribe. Ralph is also discouraged that only he and Piggy seem intent on keeping the fire going. Upon hearing Piggy's suggestion, Ralph agrees to let the fire go out. However, Ralph thinks it would be ideal to have the fire going all the time. The fire also gives Ralph a sense of security, especially at night: Ralph stood up, feeling curiously defenseless with the darkness pressing in.
they went after the beast but jack decide to forget the beast and play rolling rock
it is ironic that Jack used fire to capture Ralph becuase be was trying to find Ralph, which he did. Also the fire caused a ship to come to the idland because they saw the jungle on fire. With the ship stopping at the island and an officer coming on the beach, saved Ralph who was coming so close to a violent death. The officer also stopped the other boys from continuing on to kill Ralph. The ship saves Ralph and alll of the remaining boys. Seeing an adult and being saved, makes all of the boys emotional.
Ralph didn't directly kill Simon, but he didn't stop Jack from doing it.