What is Roger in the book Lord of the Flies?

Roger is described in the book as, and I quote... There was a slight, furtive boy whom no one knew, who kept to himself with an inner intensity of avoidance and secrecy. Roger is a sadist who takes pleasure in inflicting pain on other boys and on pigs. He is the personification of true evil. He knows what he does is wrong but he simply doesn't care, in fact he might even enjoy his actions even more because of this.