One of the main areas of conflict in the story is between the friendship that Jimmy Wells feels for Bob and the duty that Wells feels as a police officer. He is forced to choose between his friendship for Bob and the fact that he knows Bob is a criminal. We see the outcome of this at the end of the story. Jimmy chooses duty (he has Bob arrested) but he feels enough of the old love that he cannot bear to do it himself.
This is the conflict between love and duty in the story -- Jimmy has to decide whether to let Bob go because they were such good friends (so good that Jimmy showed up to keep their meeting) or whether to do his duty as a law officer.
After Twenty Years derives much of its impact from the way in which it is told, and notably from the author's ability to fuse several points of view and to join tales from periods years apart into a single narrative with its own internal logic.
The style, while often richly descriptive and evocative, is terse, and the mannerisms of the two protagonists who tell their own tales blend imperceptibly with the narrator's anecdotal approach which would not have been possible but for the point of view, hence the importance.
Jimmy is portrayed as the protagonist of the plot. He has been shown as somebody who is torn between his affection for his friend and the call of duty. In a roundabout way, he has been given the subtle role of the antagonist of the plot as well. Because the only "evil thing" of the plot, so to say, was the arrest of the "friend" who came to visit an old pal his. Jimmy proved to be a person of mettle in the end who preferred duty to friend.
However, Jimmy's actions are justified in the narrative as "Silky Bob" turned out to be a thief. But, as "Silky Bob" too kept his promise, he too, has been shown in a positive light, notwithstanding his profession.