Great question. Well, it is only at the end of the story that we realise that both of the characters are actually equally at the mercy of the other. The unnamed narrator has been working on the assumption that his connection with the rebels is unknown by Captain Torres, however, at the end, as he leaves, Captain Torres reveals that he knew about his associations all along:
In the doorway he paused for a moment and said, "They told me that you'd kill me. I came to find out. But killing isn't easy. You can take my word for it."
However, equally, Captain Torres is in an incredibly vulnerable position putting himself in the hands of the narrator for a shave. The massive internal conflict of this story is the narrator's decision of whether to kill Captain Torres or not. As he reflects, nothing could be easier than to kill him:
I can turn my hand a bit more, press a little harder on the razor, and sink it in. The skin would give way like silk, like rubber. there is nothing more tender than human skin and the blood is always there, ready to pour forth.
Thus, both characters, although the narrator does not know it until the end of the story, are equally at the mercy of the other, and both need to choose how to respond to the strange situation they find themselves in. This is what makes this story such an excellent example of suspense, as we are kept on the edge of our seats waiting to see what will happen.