Why does Ralph believe that the savages will not let him alone?

I assume that you are talking about the beginning of Chapter 12 here.

Ralph is lying there wounded, thinking about the prospects for his future.  He tries to convince himself that they will just let him alone, but he doesn't believe it.

Instead, he realizes that what the savages have done, the evil they have unleashed, is essentially out of control -- it is covering the island and will not go away.  He also feels that there is too much rivalry between him and Jack and Jack will not just want to let him get away.