What does the last paragraph of the story "After Twenty Years" mean?

The last paragraph of "After Twenty Years" is a letter from one old friend to another.

In this letter from Jimmy Wells to Bob, alias "Silky" Bob, the old friend who was supposed to have met Bob in a twenty-year reunion, identifies himself. He tells Bob that he was, in fact, at the restaurant where they always ate together and met him in the doorway at the appointed time. However, when Bob lit a cigar, Jimmy, who was wearing his policeman's uniform, recognized his old friend as the wanted man whom the authorities in Chicago were seeking.

Jimmy writes, "Somehow I couldn't do it myself," which means that he just could not arrest his old friend. So, he returned to the precinct and asked a plain clothes policeman to make the arrest. It is this officer of the law who has handed Bob this note.

Interestingly, Jimmy makes no personal comment to Bob about his criminality, nor does he express any feelings he has for Bob. This note is probably his final gesture of friendship, given out of respect for their past together.