The conflict of this story is the barber's conflict with himself. He has to decide whether or not he should take the chance to kill his customer, the captain of the military police force who has captured the barber's friends - a group of revolutionaries. The theme here is one of morality. Should he avenge his brothers, even if it means committing murder? He would be considered a hero by many, but he would think of himself as a coward and as a cold-blooded murderer. He even fears that this murder will corrupt his soul, even if for a good cause:
“Others come along and still others,” the rebel in barber's gown thinks to himself, “and the first ones kill the second ones, and they the next ones and it goes on like this until everything is a sea of blood.”
He decides to rise above the captain, to be morally better:
“You are an executioner and I am only a barber.”
It is the right choice, as the captain was prepared for an attack. H e tells the barber as he leaves that he [the captain] was told of the barber's affiliations. In the captain's words, the theme of morality is emphasized:
"But killing isn’t easy. You can take my word for it.”
Despite the captain's careless story about hunting down the rebels, he makes it clear that these deaths haunt him.
Actually, sullymonster i think you have overlooked something. There are lots of evidence that the barber is actually rationalizing, reassuring himself. -"Out of his neck a gush of blood would spout onto the sheet, on the chair, on my hands, on the floor. I would have to close the door. And the blood would keep inching along the floor, warm, ineradicable, uncontainable, until it reached the street, like a little scarlet stream."- Big deal! Just get a mop and clean it up! He has killed so many of you! Killing them for his "amusement". Even more, -"But what would I do with the body? Where would I hide it?"- Oh well. You'll find somewhere. It's a bloody war for God's sake! You can find more evidence if you look carefully through the story. -"His hands started to tremble", "I would have perferred that he hadn't come."- Well, obviously he doesn't want him to come because he is the rebel's enemy, but still! His fear is evident enough. He realizes that everybody has their own role in life, and it is hard to go pass those limits.