One of the first times we see Roger separately from the other boys on the island is in chapter four of William Golding's Lord of the Flies. He is not particularly nice from the beginning, and he certainly is not kind to the littluns now. He and Maurice leave their shift at the rescue fire and they see Henry and some littluns have built some sandcastles.

Roger led the way straight through the castles, kicking them over, burying the flowers, scattering the chosen stones.

Soon Henry is sitting alone in the sand, and Roger uses him as a kind of target practice as he first stands behind a tree and then eventually moves into the open. 

Roger stooped, picked up a stone, aimed, and threw it at Henry— threw it to miss.

Soon, however, he throws more stones.

Yet there was a space round Henry, perhaps six yards in diameter, 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 parents and school and policemen and the law. Roger’s arm was conditioned by a civilization that knew nothing of him and was in ruins.

This is significant because it demonstrates that Roger is still constrained, at least a bit, by the rules and laws of a civilized society. That has changed, however, by the time Piggy has come to Castle Rock to retrieve his glasses in chapter eleven. When Roger and Jack get tired of hearing Piggy complain and lecture them, Roger is standing above Piggy and acts as executioner.

High overhead, Roger, with a sense of delirious abandonment, leaned all his weight on the lever.

In a moment, Piggy and the conch are both crushed, and all pretense of civilized behavior is gone. Jack's tribe of savages controls the island and every boy but Ralph now belongs to that tribe. 

Things get worse, though, and we have some clear indications that Roger is even more cruel and savage than Jack. In fact, at the end of chapter eleven,  

Roger edged past the chief, only just avoiding pushing him with his shoulder. The yelling ceased, and Samneric lay looking up in quiet terror. Roger advanced upon them as one wielding a nameless authority.

The near shoulder-bumping with Jack is even more foreboding than the cruelty he is about to wield on the twins. We know how savage Jack has become, and now it is evident that there is one who has become even crueler. 

In the last chapter, Ralph manages to meet Samneric secretly, and they are clearly terrified at Ralph's suggestion that the twins try to escape with him.

“You don’t know Roger. He’s a terror.”
“And the chief—they’re both—”
“—terrors—”
“—only Roger—”

Clearly Roger is now an even bigger threat to everyone on the island than Jack. Both are bad, but Roger is somehow worse. It is Roger, not Jack, who has sharpened a stick at both ends, preparing to make Ralph the next sacrificial murder victim on the island. Roger represents the worst savagery which might happen on the island, and Golding makes it evident that Roger is on a descent into savagery that even surpasses Jack's savagery. There is little doubt, then, that if the boys had not been rescued, Jack, who seems untouchable, would eventually have had to fight for his own life against Roger.

Throughout the course of the novel, Roger transforms from a choir boy to a savage murderer, more evidence of Golding's belief that human nature will devolve into savagery without the restraints of civilization.