The antagonist of this particular story could be argued as many things depending on your personal interpretation of the literature; this is one of the reasons that this story is so great. The antagonist could be the changing of time. Emily is a character caught in the past. She cannot let go of her past, as it is shown with the dead body of her father. This is shown in the opening description of her house that stood "on what had once been our most select street". The rest of the street had industrialized and moved on and filled with cotton gins and garages, but "Miss Emily's house was left, lifting its stubborn and coquettish decay above the cotton wagons and the gasoline pumps - and eyesore among eyesores". Homer Barron is a construction worker, meaning he works to advance buildings and update the past. He is also a northerner. Put these two together and you have a character who comes from the northern states to makes a living reconstructing a southern town. Miss Emily is stuck in a glorious past that predates the war.
Emily Grierson is an isolated and eccentric Southern woman who lives on the social fringes on her town. Homer Barron is similar to her in this respect because he is a construction worker from the north. Like Emily, he is an individual who is foreign to this community, and he is subject of much town gossip as well. Homer becomes Emily's first lover. However, Emily kills Homer with the arsenic that she buys in the third portion of the narration. She exercises a kind of control and dominion over him in doing so, ensuring that he can never leave that relationship. Because Emily murders Homer, she can be considered as the antagonist of the story.