I would tend to separate out Golding's focus on the "darkness of man's heart" and the nature of evil itself. Golding does, after all, dedicate a large section to the development of Simon's "Lord of the Flies." Lord of the Flies is Beezlebub.... Beezlebub is Satan. Satan is the Judeo-Christian representation of evil. In Christian religion, Satan is evil, and "possesses" the body of the human, tempting or forcing that person to perform immoral acts.
In the novel, the boys are being possessed by evil. The make a slow progression from good to bad, demonstrating that they are not evil to begin with. The killing of the pig is when evil begins to take over, hence the introduction of Simon's "Lord". The killing of Simon - the symbolic representation of innocence and purity - is when evil succeeds.
The true chilling revelation, however, is the full emergence of Roger’s character. Previously a strange loner, his personality has emerged as truly sadistic. Even more so than Jack, Roger loves the hunt for the pain he can inflict. He slowly drives his spear into the anus of the sow, torturing it more than killing it. This is the brutal extension of his previous torture of Henry on the beach. The Lord of the Flies represents Satan, and Roger represents one of his minions.
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)