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He enlists in the army he is overwhelmed. The burden of what he saw and the stress of the war are too much for him to bear and he goes mad. Leper can't handle the aggressiveness and ruthless discipline the army inflicts.
He becomes mentally unstable.
"Leper entered the war with only an idealistic sense of what it would be like. The reality, it now seems, might have driven him insane. Leper’s fear about returning to civilian life represents all soldiers’ fears and troubles returning to society after war.
The army has made Leper grow up, though in the process it damaged him: he can admit painful things to himself now, though they make him crazy. He can see Gene’s savagery now, while Finny still refuses to.
Leper’s insanity is a fear of things transforming, of things changing. It is a fear of growing up. When children look at the world it makes sense. But as they grow into adulthood, their idealism about the world is stripped away, and it ceases to make sense or be beautiful."