What are examples of the innate nature of man in "Lord of the Flies"?
"The darkness of man's heart", as Golding puts it in the final paragraph of the novel, refers to Golding's theory that mankind is instinctively, natively, nasty and vicious. No matter what happens to man, he will be cruel to his fellow man, and that nastiness will break out.
Ralph is the key example of the innate nature of mankind, because he is the Everyman figure. He joins in with the other boys in teasing Piggy (it's him, of course, who announces the name "Piggy" to the other boys). He joins in with the mock pig hunt, which leads to Robert becoming hurt ("the desire to squeeze and hurt was over-mastering", Golding narrates from Ralph's perspective). He joins in with Simon's murder - another hunt. Like hunting, like seeking out food, cruelty is innate. It's just how we are.
As far as what Golding puts forth in "Lord of the Flies," several characters are interesting examples of the innate nature of man. Jack usually comes to mind first, as Golding uses him to establish the concept that we innately have an evil side to us, one that can surface once civilization's rules have vanished. This is clear with Jack, the anarchist, who fairly quickly wants nothing to do with any rules, and progressively grows more and more violent, leading to murder. Ralph is the somewhat more "Everyman" character in the story, however. Because Ralph tries really hard to do the right thing by society's standards, yet still succumbs to his own evil side (by taking place in the murder of Simon, for example), he represents the duality of mankind.