Ralph never understands why Jack feels he must kill him. When he confronts Jack at Castle Rock, he knows it will be dangerous, but he doesn't expect the blatant murder of Piggy and Jack's utter failure to feel remorse. Jack endorses the killing. Wounded by Jack's spear, hiding in the forest, Ralph considers the fact that there is an "indefinable connection" between himself and Jack, meaning that Jack would "never let him alone; never." But he immediately tells himself that the boys aren't that bad; that Piggy's death was an accident. He imagines going up to the tribe and saying, "I've got pax," and being welcomed into the group. But he knows he is now an outcast, "'cos I had some sense."
Still, he cannot grasp the fact that Jack would deliberately kill him. He sneaks up to Samneric while they're on watch and presses them for an answer about Jack's plans. The twins have a hard time expressing the horrible truth to their former chief; they say, "They hate you, Ralph. They're going to do you." That obviously means murder, but Ralph can't accept it, and he presses further. Finally the boys explain that Roger and Jack are "terrors" and that Roger has "sharpened a stick on both ends." Ralph cannot fathom the meaning of that warning. It means that they intend to kill Ralph like they would kill a pig, possibly by driving one sharp end of the stick up his rectum. Then they would behead him and place the head on one pointed end of the stick and plant the other pointed end in the ground, as is their practice with pigs. Ralph "tried to attach a meaning to this but could not." Later he considers it further: "What did it mean?" Even when the boys have set the island on fire, Ralph can't comprehend their true intent. He thinks, "What could they do? Beat him? So what? Kill him? A stick sharpened at both ends." Although Ralph runs for his life and knows Jack is hunting him, even to the last he doesn't understand why he "must" die or that Jack could be capable of the utterly savage act he is planning.
Ralph had initially come to Jack's part of the island to work out some kind of peace plan. However, Ralph's realization that, in Jack's mind, he is a rival who must be eliminated becomes clear after the death of Piggy. Ralph realizes how savage Jack and his gang have become. That's why he runs away. Finally, after Samneric tell Ralph about Jack's plan to kill him, if he had any remaining hopes of making peace, he must realize the impossibility of making any sort of truce. That's why he turns and runs so quickly. However, Ralph's realization of Jack's true intentions are so slow in coming, that it takes the arrival of the British navy to save him.