Ralph knows that the boys won’t leave him alone because they are becoming more and more savage—especially Jack.
Ralph is aware that the boys are no longer the boys he knew. He realizes that they have each become a “savage” under the influence of the island’s lack of accountability. He tries to argue with himself that they will leave him alone.
But then the fatal unreasoning knowledge came to him again. The breaking of the conch and the deaths of Piggy and Simon lay over the island like a vapor. These painted savages would go further and further. (Ch. 12)
Ralph is aware that Jack is his sworn enemy. The two share an “indefinable connection” that means that Jack will never leave him alone. As long as Jack and Ralph are both on the island, the island is not big enough for the two of them. The boys have killed now. They have tasted blood. There is going to be more bloodshed and violence.
In this last chapter, Ralph runs for his life and is only rescued at the last moment. The officer who rescues him does not understand what has happened. Faced again with reality, Jack’s power is lost.
A little boy who wore the remains of an extraordinary black cap on his red hair and who carried the remains of a pair of spectacles at his waist, started forward, then changed his mind and stood still. (Ch. 12)
Still carrying Piggy’s glasses and barely civilized looking, the veil has been lifted for Jack. When returning to the real world, he is unable to maintain the guise he had on the island. He is can't say he is the leader, and does not say anything at all. He stands aside and lets Ralph take charge.
This ending is symbolic. The reader is thrown into the juxtaposition of the heathen world the boys have made and the reality of the civilized world they left. Ralph will never be the same, and neither will Jack or the others.