How is Larry's letter instrumental in forcing Keller to realize his guilt?

Larry's letter is from the day he died. He had just seen the papers and heard about his father and the planes crashing. Larry felt full of guilt and anger, and wrote that he could not face anybody. He wrote that he was about to go out on a mission and that he would be reported missing. The letter implies suicide. Larry's letter to Ann adds that he loves her but that she must not wait for him.

Keller is quiet. He understands. He calls for the car and is ready to go upstairs to get a jacket.