From the very beginning of the novel, Ralph is determined to keep a signal fire going, in case a ship passes near to the island. That's all well and good, until the first signal fire the boys light begins burning out of control, and at least one boy is missing (read: burned up). As Piggy tells Jack, "You got your small fire all right" (2.210). The fire thus becomes a symbol, paradoxically, of both hope of rescue and of destruction.
Ironically, it is because of a fire that Jack lights at the end of the novel—in his attempt to hunt and kill Ralph—that the boys are rescued. And it makes sense. If the boys' world is just a symbol for the real world, then they're not being rescued at all; they're just going on to a larger scale of violence—to grow up into soldiers getting sent off to war. Hence, rescue equals destruction.