This guy is the only Nazi with a name in this story. That alone is a pretty big deal: putting a name on a possible antagonist makes him all the more real. He's someone toward whom we can direct our anger, right? But we don't know much about him, except that he's the one in charge of determining Jaromir's fate.
Having read a catalog from a publishing house that exaggerated Jaromir's reputation as a Jewish writer, the Captain decides to put him to death. Although this decision could be seen as the act of a monster, he's not really characterized that way. Instead, Borges emphasizes the ways in which he's just like everybody else.
Everyone, the narrator says, is pretty gullible when it comes to information that's outside their area of expertise. In fact, the Captain is just working as anyone – or anything – would:
That delay (whose importance the reader will soon discover) was caused by the administrative desire to work impersonally and deliberately, as vegetables do, or planets. (2)
But we can't really compare Captain Rothe's job to the impersonal work of a vegetable or a planet. Vegetables and planets can't do evil – people can. And so Captain Rothe's character raises the question: why do regular people do really terrible things?