What is the theme in Lord of the Flies by William Golding?

There are a number of themes that you can consider.

The theme of killing a pig for the sake of food vs killing thing for the joy of killing comes into sharper focus. Golding feels that it is inherent in man to impose his will on other creatures.

The theme of the irrationality of fear becomes clear as well. Ralph discovers that he can't merely legislate and end to fear of the "Beastie".

The theme of Simon as a Christ figure who seeks solitude also becomes clearer by his extended forays into the jungle alone.

Huts on the Beach is rather ironic. THe boys can't seem to get the simple act of building a shelter done. Instead, like children, they satisfy more primal desires (hunt, play....).