One of the big questions raised by Lord of the Flies is whether the boys in their primitive actions are reverting to a somehow inferior state of life, or whether they are driven to their natural and rightful states. If well-brought up British boys become violent savages when left without supervision, maybe people really are just violent savages, covered up in clothes and caps. But big questions aside, primitivity in Lord of the Flies means hunting, the desire for food, the desire for power, bloodlust, violence, sadism, and a general inability to distinguish between man and beast.
Though Roger and Jack commit similar actions, it is the difference in their motivations that renders Roger morally inferior to Jack. While Jack is driven by a desire for power, Roger is compelled by purely sadistic impetuses.