What implies that Roger was a bully by nature and Maurice only by imitation?

Roger and Maurice are two of the boys who are older than littluns but are not among the oldest of the boys. In chapter 4, they come down to the beach together after having been relieved of their duty of tending the signal fire. Although each boy behaves as a bully to the littluns playing on the beach, Roger's actions show him to be naturally mean, while Maurice is more of an imitator of bad behavior rather than an instigator.

Roger immediately destroys the littluns' sand castles as he walks past. He then stays and keeps watching the littluns play. He has a "gloomy face" with an "unsociable remoteness." He follows one of the little boys, Henry, and while Henry plays at the water's edge, Roger throws stones all around him, leaving a space of about three yards on either side. Roger obeys an "invisible yet strong ... taboo," but does not seem to have any internal compunction against hurting someone. When he has a chance to kill a pig during the hunt, Roger is the one who finds the vulnerable spot on the pig and "began to push till he was leaning with his whole weight." Roger's is the effort that actually kills the sow. When the four boys go to Castle Rock in chapter 11, Roger begins throwing stones at Samneric. First he aims to miss, but then "some source of power began to pulse in Roger's body." Then Roger starts throwing stones at Ralph, close to his head. Finally, "with a sense of delirious abandonment," Roger leans on the lever and sends the huge boulder rolling toward Piggy, knocking him to his death. Sam describes Roger as "a terror." The fact that he "has sharpened a stick at both ends" is Ralph's death sentence in the final chapter, for Roger has given himself over fully to his natural vicious tendencies.

Maurice, on the other hand, after having destroyed the sand castles and getting sand in Percival's eyes, feels "the unease of wrongdoing." He leaves the children to play and runs away for a swim. At the end of chapter 4, when the group of hunters reenacts the pig kill, Maurice "pretended to be the pig and ran squealing into the centre." Later in chapter 5 when several littluns start crying during the meeting, "Maurice saved them." He clowns around, pretending to fall over, and gets the boys laughing. Maurice obviously has a naturally imitative personality, but he cares about the feelings of others.

Maurice represents many of the boys who, although they are not overly malicious themselves, allow themselves to become part of the malicious regime of Jack and Roger. Roger represents the boys who have a naturally power-hungry nature and act on it, eventually leading the boys into murder and near annihilation.