What's Roger's role in chapter 11 of Lord Of The Flies?

Roger acted as a sentinel at the Castle Rock where Jack and his tribe had set up a fort. It was his job to sound the alarm when Ralph or any of the others approached. His duty was made clear when Jack at one point asked him why he wasn't on watch. He explained that he had just come down, but did not complete his sentence.

Roger was one of the older boys and Jack's right hand. He therefore could be deemed second in command. He was one of the choirboys and was a member of Jack's hunters. Throughout the novel, he is painted as a dangerous character, one with an almost instinctive leaning towards evil. He was secretive and seemed not to like attention as illustrated in the following extract:

There was a slight, furtive boy whom no one knew, who kept to himself with an inner intensity of avoidance and secrecy. He muttered that his name was Roger and was silent again.

Golding consistently paints him as a sinister character who is quite cynical as in the following examples:

The dark boy, Roger, stirred at last and spoke up.

Roger took the conch and looked round at them gloomily. “I’ve been watching the sea. There hasn’t been the trace of a ship. Perhaps we’ll never be rescued.”

Roger is also deliberately destructive and seems to enjoy scaring and even hurting others. He intentionally destroys the littluns castles and later throws stones at Henry whilst hiding from view.

He was not noticeably darker than when he had dropped in, but the shock of black hair, down his nape and low on his forehead, seemed to suit his gloomy face and what had seemed at first an unsociable remoteness into something forbidding.

When he and Jack capture a pig, it is Roger who seems to relish the idea of inflicting pain on the helpless creature:

Roger found a lodgment for his point and began to push till he was leaning with his whole weight. The spear moved forward inch by inch and the terrified squealing became a high pitched scream.

It is therefore hardly surprising that he is responsible for Piggy's horrific death. Roger had intentionally pushed on a lever to dislodge a large rock which bounded down, first shattering the conch and then knocking Piggy over the ledge, hurtling him forty feet down to his demise.