Although it is tempting to view Emily's murder of the Yankee, Homer Barron, as the South smiting the wicked North for its transgressions of the Civil War, there are several reasons for not yielding to that view, not the least of which is that Faulkner himself explained that although the North and South are, in a sense, symbolized by Barron and Miss Emily:
. . . but it was no intention of the writer. . . . he was simply writing about people, a story which he thought was tragic and true, because it came out of the human heart, the human aspiration, the human--the conflict of conscience with glands, with the Old Adam. (1959 interview, "On 'A Rose for Emily'").
Faulkner acknowledged that the environment of the story--a small southern town in the years following the Civil War--contained the elements of a North-South controversy, but those elements were incidental to the narrative, not the framework. In the interview, Faulkner went on to say that the conflict was not between "North and the South so much as between . . . God and Satan," so we can reasonably argue that the North and South are represented in the story, but they are not at the heart of this story.
Barron, as a blue-collar Yankee worker, is necessary to the story not because he's a Yankee but because he is as outside the world of Miss Emily as a person can be. The only person in this story who can reasonably be expected to begin a relationship with Emily is someone from far away, both in terms of geography and in social status. Barron fits his role precisely because he is the lowest of the low in the town's eyes, a Northerner and a manual laborer, and, because he is an outsider--a stranger in a strange land--his inclination is to ally himself with someone whom he believes is still respected because she is a remnant of southern aristocracy. He has no notion, for example, that Miss Emily is perceived by the town as a "fallen monument."
Emily's attraction to and eventual murder of Barron has nothing to do with his background or his Northern-ness but everything to do with his availability and liveliness, at the beginning, and his clear unsuitability for marriage, at the end. In other words, Emily is attracted to him because he represents something she lost, or never had, when she young, but when she discovers who he really is--not the marrying kind--her response is to hold onto him in the only way possible, frozen in time (that is, dead). Emily is, in short, attracted to the idea of Homer Barron, not the reality.
Homer Barron represents, even in death, a relationship that was denied to Emily earlier in her life, and the only way Emily could maintain that relationship--in her oppressed, repressed, twisted mind--is to keep Barron with her alive or dead. Unfortunately, keeping him alive is not a possibility.