The Federal soldiers who execute Farquhar are the embodiment of military justice. They act with precision and harshness, and whether or not we agree with their actions, they're the antagonists because they act against the protagonist. Think back (if you can) to your hours spent shooting nameless hoards of soldiers/zombies/aliens while playing video games. The figures you shot probably had no names, no thoughts, and no background. In the world of the story, Farquhar's executioners are similar. However, they are also members of an army fighting to reunite the Union and free the slaves. Nevertheless, they seem programmed to destroy Farquhar and that is their only role in the story.