Leper enlists in the US Army after watching a recruitment film that glorified the nature of service in the Army. While Leper is the least likely student to enlist, the film convinces him that army life will be amenable to his personality.
...he is the first student in the novel to en-list in the Army, because a deceptive recruiting film convinced Leper that Army life is a clean, pure experience. (eNotes)
Leper goes AWOL soon after arriving at boot camp and runs away to his home. Leper realizes a few truths about himself and others during this time and comes to see the world with some bitterness. Gene discovers this when he goes to visit Leper.
Leper calls Gene "a savage underneath" and presents a quite different demeanor than he once had during this visit, lashing out at Gene, his only "friend".