What the arresting officer seems to be implying is that Bob was a good man twenty years ago but that he has changed into a bad man during that time. It seems plausible that Bob was a "good" man twenty years earlier because he and Jimmy were almost like twins when they parted at 'Big Joe' Brady's restaurant. As Bob tells Jimmy (without knowing who he is talking to):
"He and I were raised here in New York, just like two brothers, together. I was eighteen and Jimmy was twenty."
This is an important bit of exposition. The two young men must have been very much the same since they were so close. Bob describes Jimmy as:
"...my best chum, and the finest chap in the world."
No doubt Jimmy thought the same of Bob at that time. They were both quite young and had not been exposed to the world long enough to acquire much in the way hardness or vices. Bob was actually two years younger than his chum, so he would have been even more naive and innocuous. Jimmy continued to be "the finest chap in the world," but Bob went out into a world full of temptations and bad companions. He acquired a smooth manner, but he was hardened and corrupted by twenty years of struggle in a raw environment. We get a hint of his character when he lights a cigar and we, along with Jimmy, have a glimpse of his face.
The man in the doorway struck a match and lit his cigar. The light showed a pale, square-jawed face with keen eyes, and a little white scar near his right eyebrow.
That sounds like the description of a hardened criminal. The keen eyes must have come from twenty years of living outside the law, watching out for danger and for opportunities. The little white scar must have come from a knife fight. In fact, it must have been the little white scar that enabled Jimmy to identify Bob as the man wanted by the Chicago police. There was no way of sending pictures by wire in those days. The message from Chicago must have contained a complete physical description of the man known as 'Silky' Bob including the important information that the wanted man had a small white scar near his right eyebrow. It might have even included the fact that "his scarfpin [is] a large diamond, oddly set." The fact that it is "oddly set" would be another good identification sign. The unusual setting for the diamond could be described in detail in the telegram.
People change. Twenty years is indeed a long time. It is long enough for one man to become a cop, while another becomes a criminal. Bob and Jimmy were old friends who took very different paths in life, but they both kept the date to return to the spot twenty years later. At first, Bob is not suspicious. He thinks that Jimmy is the man he is talking to.
Jimmy is a very devoted cop. Integrity is important to him.
Trying doors as he went, twirling his club with many intricate and artful movements, turning now and then to cast his watchful eye adown the pacific thoroughfare, the officer … made a fine picture of a guardian of the peace.
He keeps his appointment, but recognizes his old friend—from wanted posters. He says nothing to him about who he is, and moves on. Then he finds a cop down the street and asks him to impersonate him.
Bob comments that twenty years is not long enough to change someone’s physical appearance. The cop he is talking to is not his old friend Jimmy after all.
"It sometimes changes a good man into a bad one, said the tall man. "You've been under arrest for ten minutes, 'Silky' Bob….”
Jimmy did not have the heart to turn his friend in. He had to get someone to do it for him. Twenty years is long enough to turn a good man into a bad one, but not long enough to turn a bad one into a good one.