Evil manifests itself in a number of ways, but the most disturbing (for me, anyway) is the propensity for evil that is a part of being human.
This take on the theme of evil can be found in a number of exchange. First, the boys, like most of us, are convinced that evil is not a part of their nature:
'We've got to have rules and obey them. After all, we're not savages. We're English, and the English are best at everything.'" (Ch. 2)
But things quickly go downhill:
He [Jack] began to dance and his laughter became a bloodthirsty snarling." (Ch 4)
By Chapter 7, all civilized nature seems to have vanished, replaced by a thirst for evil: "'Kill the pig! Cut his throat! Kill the pig! Bash him in!'" and "Ralph...was fighting to get near....The desire to squeeze and hurt was over-mastering."
By Chapter 12, it is all too clear how much evil a part of their make-up: "Ralph launched himself like a cat; stabbed, snarling, with the spear, and the savage doubled up." Ralph himself learns this painful lesson:
""Ralph wept for the end of innocence, the darkness of man's heart, and the fall through the air of the true, wise friend called Piggy."
(Of course, you could also explore the theme of mythical the "beast" in the novel and how it relates to evil as well)