In Lord of the Flies, why is Roger more sadistic than Jack?

In this allegory of "The Lord of the Flies," Roger and Jack are analagous to Ralph and Piggy in the sense that of the two pairs of boys, one is more representative of a "pure" trait that the other. That is, Piggy and Roger are more allegorical characters, representing qualities rather than rounded characters.

For instance, Piggy is purely rational while Ralph cannot always think as rationally; for example, he participates in the hunt for the pig at one point in the narrative.  Roger, numb to the suffering of others, represents pure savagery from the beginning to the end, while Jack displays more human characteristics at the first, hesitating at first to stab a pig, for instance, but later degenerating into savagery once he can hide behind the mask of the paint on his face.  Early in his novel (Chapter 4), Golding depicts Roger as intrinsically evil:

[As Henry plays in the shallow water as the shore] Roger waited too. At first he had hidden behind a great palm; but Henry's absorption with the transparencies was so obvious that at last he stood ut in full view....Roger stooped picked up a stone, aimed, and threw it at Henry--threw it to miss.  The stone, that token of preposterous time, bounced five yards to Henry's right and fell in the water...there was a space round Henry...into which he dare not throw.  Here, invisible yet strong, was the taboo of the old life.  Round the squatting child was the protection of parnet and school and policemen and the law.  Roger's arm ws conditioned by a civilization that knew nothing of him and was in ruins.

It is Roger who "carries death in his hands" because he is pure predator, only "conditioned by a civilization."  And, once that civilization loses its hold, Roger gives free reign to his intrinsic sadism. He releases "with a sense of delirious abandonment" the rock that kills Piggy, and he intimidates SamnEric into joining the hunters' group. as he "advanced upon them as one wielding a nameless authority."