By the beginning of Chapter 12 of Lord of the Flies, all of the boys who were loyal to Ralph are either dead or have joined Jack's tribe. Both Simon and Piggy were murdered by Jack's group, and the others have all gone over to the other side or been taken prisoner. Additionally, the conch has been destroyed--the last remnant of civilized society. Ralph is all alone. He tries telling himself that Jack will let him be or allow him to become an outcast, but he realizes that the boys' savagery would "go further and further." But it is the rivalry with Jack--their battle for supremacy of the boys--that worries him most.
... that indefinable connection between himself and Jack; who therefore would never let him alone; never.