The story is an intense meditation on a moral dilemma: should the barber murder the murderer who has come to his shop for a shave? Told from the point of view of the barber, the tone of the story is one of anxiety and suspense: as the barber tries to decide what to do, the reader also wonders what the outcome will be.
This tone is heightened as the story progresses and the barber engages in a kind of internal monolog about his responsibilities as a rebel sympathizer and the ethical responsibilities of his profession. While it would be easy to kill the captain, he also knows that he would be hunted down and killed himself if he did so. He tries to justify his inaction by arguing that his role is to pass on information, not kill people. Finally, he reflects that, although the Captain is a cold-blooded killer who stages public executions and then mutilates the bodies, if he were to kill him he would become like the captain himself: another cold blooded killer.
The relief the barber feels once this decision is made is palpable. However, the end of the story provides a twist that changes both the meaning and tone. The captain knew all along that the barber might kill him, but he went for a shave anyway, trusting that the barber would not have the courage (or the ability to suppress his humanity) to do it. The anxious tone returns, then turns to one of shock. The barber knows that he has been found out as a rebel sympathizer. While there is some relief that the captain seems to have spared his life anyway, there is also a final sense of dread about what might happen to the barber in the future.