What quotes show that Roger is a victim of Jack?

Roger is not a victim of Jack but is in full agreement with Jack's methods and uses them to inspire his own brutal conduct. Early in the story after Maurice and Roger finish their stint as fire-tenders, they head to the beach. Roger immediately terrorizes the littluns on the beach, destroying their sand castles. While Maurice exhibits a sense of conscience for making one of the boys cry, Roger doesn't. He proceeds to tease Henry by throwing stones at him from behind a tree, aiming to miss. When Jack shows Roger the face and body paint, "Roger understood and nodded gravely."

During the sow hunt, Roger pushes past Jack and inserts his spear into the sow's rectum, driving it further and further in. Jack selects Roger to be one of the boys to steal fire from the others, and Roger doesn't object. During Jack's feast, "Roger became the pig, grunting and charging at Jack, who side-stepped." This is not the action of a victim, but rather a partner. When Jack makes plans to steal fire again, this time Piggy's glasses, Roger volunteers to go with him.

When Roger comes to Castle Rock and sees that Jack has created a defense that would "send the rock thundering down to the neck of land," Roger approves, saying, "He's a proper chief, isn't he?" Later, when Ralph and Piggy come to Castle Rock, Jack is away hunting, and Roger is manning the defenses of the camp. He begins throwing stones at the twins, "aiming to miss." But then "some source of power began to pulse in Roger's body." Later he begins zinging rocks very close to Ralph's head. When Roger sends the rock down on Piggy, it is from his own "sense of delirious abandonment," not from Jack's order. When Roger sees Jack merely prodding the captured Samneric, he declares, "That's not the way," and evidently causes the twins actual physical harm, causing them to tell Ralph, "He's a terror." In the final hunt for Ralph, it is Roger who has "sharpened a stick on both ends," presumably in order to kill Ralph the way he killed the sow. Throughout the story, Roger's acts of cruelty are the result of his own choices, not a submission to Jack's influence.