Roger's an interesting character, whom Golding uses to represent (in a rather unequivocal way) the 'darkness of man's heart' which the novel is so interested in.
When first introduced, he's marked out as a "dark" figure from the word go:
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.
What 'secret' does Roger hold? That inside, he's got a sadistic desire to cause pain to the other boys. The next key moment comes when Roger is throwing stones at the littlun Henry, hiding behind a tree:
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.
Roger, at the start of the novel, is still conditioned by the rules and civilisation of the outside world. But as the novel goes on, his sadism - his enjoyment of causing pain - becomes more and more clear. This is from the murder of the sow:
Roger ran round the heap, prodding with his spear whenever pigflesh appeared. Jack was on top of the sow, stabbing downward with his knife. 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 highpitched scream.
Roger's spear, we later find out, has gone 'right up her ass'. It's a brutal act: and it's echoed later in his murder of Piggy (when he leans on the lever and tips the rock with 'delirious abandonment') and his eventual role as Jack's chief torturer (Samneric describe him as a 'terror').
What's the important of Roger? He is a manifestation of the 'blackness within'. He enjoys causing pain, he is cruel, he tortures the other boys, he is sadistic. He is exactly the sort of person who civilisation needs to exist in order to restrain.
Hope that helps!