At the end of the novel, in Chapter 11 and 12 Roger does several different things. In Chapter 11 Roger is the boy who uses a stick as a lever and causes the rock to roll down the hillside. Even though Ralph just gets out of the way, the boulder knocks Piggy off the cliff and he falls to the rocks below.
In the following chapter, Sameric tell Ralph he must run away because they are going to hunt him in the morning. They tell Ralph that Roger has sharpened his stick on "both ends."