If I were to write a monologue from Captain Torres' perspective in Hernando Teliz's short story, "Just Lather, That's All," I believe I would familiarize myself with what a monologue is, and then begin by studying the barber's monologue.
Dr. L. Kip Wheeler writes:
MONOLOGUE: An interior monologue does not necessarily represent spoken words, but rather the internal or emotional thoughts or feelings of an individual
With this in mind, it is understood that your monologue will be similar to the barber's in that it will be an internal "discussion" that Torres has with himself. However, the barber's monologue deals with his dilemma of killing an enemy or stopping the chain of violence—refusing to be like the vicious and violent Captain Torres. Torres' monologue will be totally different.
If I were completing this assignment, I would reread the story, making notes or highlighting characteristics and attitude of Torres. I would try to find a logical place to insert his monologue which is very important based on the story's conclusion. Torres expected the barber to kill him, and still he sat in the man's chair, eyes closed and throat exposed.
This information is well worth noting. Torres is a murderer; he is violent beyond belief. He is also very brave or extremely overconfident. These things bear consideration if your monologue is to be believable.
I would probably begin by thinking of the success he has just experienced with the band of rebels he has captured. Perhaps he wonders what he will do with them, or wonders if he will receive a promotion...or if he should worry about an uprising. Perhaps he is also tired. He might wish for something cold to drink and a shady, quiet spot. At some point, he might notice the barber's shaking hands and wonder what kind of man the barber is, beyond his profession. Perhaps he would wonder about the kinds of things he hears or is told while shaving customers. The knowledge that he knows about the threat the barber poses and still sits in his chair would have to come at the very end in order for the story to still surprise the reader.
If you study the characteristics of Torres and stay focused on a realistic depiction of the man, you should do fine. Let the story guide you in this respect.