The most significant event that Roger is most responsible for is the death of Piggy. He is atop a cliff guarding Jack's tribe from any trespassers when Piggy approached. Roger pushed a boulder down the cliff that directly hit Piggy and eventually killed him.
Roger is also a right hand man to Jack throughout the novel. One of the stronger boys, he is helpful in the hunts and follows Jack's lead.
Roger encourages the mock session to kill the pig when the group took Robert as their sample pig. This session grew terribly close to an actual kill. Roger seems to represent the desire to kill. All of the events he has influence over seem to either insinuate death or end in death.