When Gene visits Leper, he learns the psychological effects that war can have upon someone, and Gene learns that the same evil that begins wars resides within himself.
After receiving the telegram from Leper and visiting him at his home, Gene finds that his own "separate peace" in which he has sheltered himself from the war is now shattered. For, he is greatly disturbed by Leper's condition as he knows he will soon be drafted himself. As the two youths talk, Leper tells Gene he was going to be given a Section Eight, so he had to "escape" because, otherwise, he would be marked for life and never obtain a job.
Suddenly, Leper tells Gene, "You were always a savage underneath...like that time you knocked Finny out of the tree." Gene reacts furiously by kicking Leper's chair over, causing him to tumble and collapse on the floor, yet Leper is still laughing, although crying, too, because he is no longer is in the circle of Gene and Finny. In much the same way that he struck out at Finny--in a blind rage--Gene strikes out at another friend, demonstrating the truth of Leper's accusation of his being a "savage underneath." These brutish actions are also at the heart of wars.
Chapter 10 is a pivotal chapter to the novel as it takes the action away from Devon School and places Gene in the real world where he is faced with the damaging effects of the war on his former friend, who claims that the army "turned everything inside out." When confronted by the fears and brutishness of his own mind in the face of these revelations, Gene does, indeed, release his "savage" and bellicose nature.