What is Roger's duty at the fortress?
Golding gives us the answer to this one just after Piggy has been killed by Roger.
Presently the tribe returned noisily to the neck where Roger joined them. The Chief spoke to him angrily. 'Why aren't you on watch?' Roger looked at him gravely.
'I just came down -' The hangman's horror clung to him.
Roger's role at the fortress seems officially to be "watchman". Though what he actually becomes is the devisor of tortures - the official torturer and sadist, who seems to take some pleasure in causing pain. It is Roger, who, at the end of a novel, has sharpened a stick at both ends - for reasons undeclared by Golding, but his killing of Piggy has already bred in the reader's mind a real fear of what he might do:
Ralph stood facing them, a little to one side, his spear ready. By him stood Piggy still holding out the talisman, the fragile, shining beauty of the shell. The storm of sound beat at them, an incantation of hatred. High overhead, Roger, with a sense of delirious abandonment, leaned all his weight on the lever.
"Delirious abandonment". Out of context, it sounds like someone on a funfair ride. Roger's official role is watchman, but his role in the novel is far more ominous - a shy, sinister, sadist.