How does the reader know that Tim O’Brien is still bothered by this incident when he writes this book?

O’Brien imagines what the man’s life must have been like. He makes up a whole fictional biography for his victim: He was always afraid to go to war, after hearing about the exploits of other war heroes in his village. He never thought he could be a hero, had avoided politics, was a scholar who loved math, was possibly in love with a classmate.