In "Just Lather, That's All," there are a number of reasons which contribute to the barber's decision not to kill Captain Torres. Here are three of them:

1. The barber describes himself as being "conscientious" and, in his world, this means that he never spills a "pearl" of his client's blood. Killing the Captain would, therefore, sully his reputation and eliminate his respectability.
2. The barber believes that murder is wrong because it goes against his principles. He identifies as a "revolutionary" and this is, in his opinion, very different from being a murderer.
3. The barber realises that murdering the Captain would create more problems than it would solve. Instead of ending the conflict, for instance, it would enable it to continue because members from both sides would use murder to bring about retribution: 

"Others come along and still others, 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."

Above all else, the barber did not want to commit the greatest sin of his profession--to spill a drop of the customer's blood. Killing Captain Torres while performing his barbering duties went against the unwritteen barber's code. Additionally, the barber considered it a cowardly act of murder, killing the captain while his eyes were closed. He believed that killing the captain in this manner would make him no better than his victim. Also, the barber knew that he would be identified as the killer--murderer or hero, he knew not which--and he had nowhere to go to hide from the men who would be hunting for him. Killing the captain would also reveal his true identity as a spy for the rebels, and by blowing his cover, he would no longer be useful as an operative.

What reasons does the barber give in favor of killing Torres in "Just Lather, That's All?"

The reasons that the barber gives in favor of killing Captain Torres have to do with Torres’s actions. Torres is a bad person who has done a lot of harm to a lot of people. The barber is an informant to the rebels who are against everything that the captain does and everything that he stands for. The barber thinks about how if he killed him, he would be helping his own cause and protecting the captain’s future victims. Consider what the barber thinks to himself while he is getting rid of the captain’s beard:

How many had he ordered shot? How many of us had he ordered mutilated?

These questions show the reader that the reasons the barber lists for killing the captain have to do with the numerous violent crimes the captain is responsible for. Once the barber is done shaving the beard, he goes on to think,

He is a calm man, who is not even thinking about what he is going to do this afternoon.

Here the barber is reflecting on the captain’s character. He is noting Torres's indifference and insensitivity to the violent things he does, which supports the option to kill him.

The barber also thinks about how easy it would be for him to kill. He notes that if he just pressed a little bit harder on the razor, then he could easily pull it off. But in the end, the reasons he lists against killing him are more convincing.