In ch.12, Does Ralph understand why he must be killed?; what is the irony of Samneric's behaviour?
The bitter irony of the story’s end is that the smoke the boys use to flush Ralph for the kill is the signal for their rescue. Jack, or perhaps even Roger, unleash their power in order to kill Ralph: the last remaining vestige of civilization. They do not seem to care, or perhaps they do not understand, that what they have done will destroy them all. Like Ralph, Jack is unable to see the other side of his own nature. Ironically, it is Ralph’s vision that is realized when a ship arrives after having spotted their smoke. Ralph knows he will die, even though he may not completely understand the reasons for his death. The only two truly non-violent people--Simon and Piggy--don't survive the island.
The irony of Samneric's behavior is that they allow Ralph to pass by them through the forest without signaling the others. They are to "hunt" Ralph like all the other boys, but they recognize that Ralph has been good to them and allow him to pass even though they realize Ralph's death is eminent.