The character Roger represents the sadist, the individual who enjoys hurting others. His evil motives are different from Jack's, who pursues leadership and stature and enjoys the thrill of the hunt. Roger just likes to hurt people.
He is described in Chapter 1 as a boy "who kept to himself with avoidance and secrecy." His secret is that he is, in some ways, more evil than even Jack. All his life, Roger has been conditioned to leash or mask his impulses. The "irresponsible authority" of Jack's reign offers him the chance to unleash his innate cruelty.
Initially, in a mean-spirited prank, Roger throws rocks at the unsuspecting littlun, Henry, but he throws them so that they miss, surrounded as Henry is by "the protection of parents and school and policeman and the law. Roger's arm was conditioned by . . . civilization." Once he joins Jack's tribe, he has lost that conditioning and eventually performs the ultimate act of intentional cruelty toward Piggy.