A fear of the unknown runs throughout the novel. To begin with, the boys don't really know what happened at the start of the novel. They have only rumors and bits of stories about a war. No one knows where they are. Therefore, no one could rescue them even if the war allowed. When the boys first gather together, they can't really keep count of how many boys there are (they don't know). When they do get together, the things that move in the dark, such as the snake, are the sources of fear. What is it that makes the boys feel like something is watching them when they are hunting? What is it? They don't know, and it terrifies them.
To give more specific examples, consider the following. At times the mirage makes distance seem to change, and no one really knows why In chapter 5, Ralph speaks of things breaking up and says that he doesn't know why. When the parachutist comes to the island, the boys can't recognize him as human. They see him as "the beast" instead. Soon after, Ralph realizes how much he can't remember about what has happened, and that they are all changing. All of these things, and more, frighten the boys.
Fear is basically the overall theme of the book.
The boys on the island are afraid of numerous things, most of them not even knowing what it is that causes the fear.
The boys were afraid of a beast (although in the end, they were right). But they didn't even physically see it so they have no idea what it actually is. The boys believed they finally killed the beast, which is actually Simon, also another example of "the fear of the unknown". They didn't know it was Simon, they were scared and their first reaction was to kill it.
The theme "fear of the unknown" runs throughout the whole book. The author provides fear to the boys through the idea of the beast lurking in the depths of the deserted island, making them frightened and scared. They started wondering, thinking, suspecting and shuddering with fear with the thought of a monster lurking in the Eden-like island. Also, different types of fears popped out when the boys stayed in the island- the fear of snakes slithering through the woods at night and also the beast stalking them. They also cannot give up the notion that the beast is living in the same island that they are living in while they also cannot give up the scary idea too, so the relationship between the hideous beast and the fear of the children seems to be in correlation.