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Roger is a socio-path. The fine line between the thin rules of civilization and reckless abandon of "nameless authority" quickly vanishes for this boy. He is content to throw stones at Henry barely missing him because of some unspoken conditioning in the back of his mind. This disappears by the time he lops a boulder on Piggy's head. He becomes Jack's "muscle" and enforcer.
I think that one more idea to consider is that, although he is not as clever as Jack, Roger knows when he is being abused and humiliated. Near the end of the book, Jack publicly admonishes Roger for not keeping proper watch. Rodger walks past Jack slightly missing pushing his shoulder on purpose. If Roger were to strike out at Jack, Jack's tenure as chief would be over. With the Alpha-boy defeated the boys would quickly turn on each other. In his own plodding way, Roger holds the balance of power.