When and why does Roger sharpen a stick at both ends?
Roger is the one boy in the group that appears to have completely accepted his own evil and is completely willing to kill or hurt in order to accomplish whatever purpose he sees as important. It is clear as he progresses from playing at being hurtful and cruel to looking forward to it.
By the time Piggy and Ralph come to the hunters to ask for the glasses, he has gone over to the dark side and relishes the chance to drop the huge rock and kill Piggy.
He sharpens the stick at both ends in chapter twelve, and it is Samneric who tell Ralph about it. It isn't entirely clear what he intends to use it for but given their attitude it would appear to be some kind of torture device.