What is the theme of the short story "Just Lather, That's All"?
In my opinion, the theme here (in the way you are describing theme) is the meaning of bravery. I think the story is about what it means to be brave as opposed to what it means to be sensible. I think the story encourages you to think about which is better. I think this qualifies as something that makes sense even outside this particular story.
In the story, the barber has to decide whether it is better to be brave (kill the guy he is shaving) or to be sensible. It is not really clear whether he makes the right choice -- it's up to us to decide.
I hope that is the sort of thing you're thinking of...
I would go for Humanity vs. Inhumanity because, at that precise moment where both men are together at the barber shop and the Captain is telling the barber, who is an informant, all about what is going on in the guerillas, the two men are at level with each other in terms of deciding morally what is the fate of another human being under the circumstances.
So, that precise second where the barber contemplates slicing this inhuman monster, his own humanity was put to the test and so the two oppositional forces of the characters bring out the theme of the story, which is that the battle of humanity vs. inhumanity of war is the causative factor of the entire conflict.
One of the themes explored throughout Hernando Téllez's short story "Just Lather, That's All" concerns destiny. As the barber is shaving the captain's beard, he wrestles with the decision to slice his throat or continue shaving. The barber begins to think about his future if he were to kill Captain Torres and contemplates how he would have to flee to somewhere far away. The barber knows that he would be chased and viewed as a coward by his opponents. However, the revolutionaries would view him as a hero and avenger. His fate rests on his decision, and he comments,
"My destiny depends on the edge of this blade" (Téllez 3).
Captain Torres also puts his destiny in the hands of the barber. He risks his life by allowing a revolutionary the opportunity to kill him. However, the barber decides to spare the captain's life and risks being called a coward by his fellow revolutionaries. The barber chose to maintain his integrity and resist temptation. By controlling his emotions, he was able to make a decision which did not drastically change his life in the way that murdering Torres would have.