Since the story is told completely from the barber's point of view, there is not much information about the captain we have that is firsthand. We know from the barber's questions that the captain has been in the jungle for days hunting rebels; we know that he compelled everyone in the town to watch the bodies of four revolutionaries be strung up at the school. We also know that the captain made the town watch when his men used the bodies for target practice. So it is fair, I think, to describe the captain as ruthless and sadistic.
The captain stands in contrast to the barber, who reflects on the violence of the captain ("a man of imagination," he says) while also contemplating cutting his throat. The barber feels a certain obligation to do his job as a barber faithfully. It is possible that he really does feel that way; his decision not to kill the captain could be an expression of honor, or a calculation so that he can continue to inform the rebels about the Captain's actions, or it could be that he is afraid of what will happen to him after. All this thinking differentiates the barber from the Captain, however, who -- apparently -- acts without remorse or a second thought.
The ending comment by the captain, which shows that he was both aware of the barber's connection to the rebels, and his own danger, is a bit of a twist, but does not really clarify what we know about him. It does seem, on the face of it, that the captain was courageous to have the shave knowing what he did. There is also a great deal of arrogance in his remark, suggesting as it does that perhaps he knew he was safe because the barber would not dare do anything. His final admission that "killing isn't easy" highlights this ambivalence. This could mean that the captain does feel some remorse about his killing; it could also mean that killing "isn't easy" for men like the barber who have a conscience (or who are not brave), and that it is a task the captain has mastered. Either way, the captain is showing the barber that he knows the barber is a rebel. One can only imagine the barber's fate.
In "Just Lather, That's All," Captain Torres is a hardworking person who is dedicated to his career. At the beginning of the story, it is revealed that he spent the last four days in pursuit of some rebel soldiers. His dedication to the job is reinforced through his description: he has a four-day beard and his face is "reddened" by the sun.
From his description in the text, it is also clear that the Captain can be cruel and violent. He allows his soldiers to use some dead rebels as target practice, for example, and he describes the rebels' hanging bodies as a "fine show." At the time of his shave, the Captain is also planning a suitably violent and gory punishment for the rebels he has most recently caught.
Finally, the Captain is also brave, as shown in the final line of dialogue in the story:
"They told me that you'd kill me. I came to find out."
The Captain knew the barber wanted to kill him, but he was brave enough to test the barber's nerve.