In Lord of the Flies how does Ralph understand why he must be killed?

To quote directly from the final chapter of the book, in which Ralph mulled his situation over in his mind... He argued unconvincingly that they would let him alone; perhaps even make an outlaw of him. 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 vapour. These painted savages would go further and further. Then there was that indefinable connection between himself and Jack; who would never let him alone; never.