Why does Emily kill Homer? Consider her upbringing, the society she lived in, and her isolation.
We'll never know for sure if or why Miss Emily killed Homer. She must have realized that Homer was her last best chance at marriage; and though Homer preferred the company of men and was a confirmed bachelor, Miss Emily must have thought she could convince him to stay in Jefferson and marry her. We'll never know if Homer made Emily any promises concerning marriage, but it is unlikely that he actually asked her to marry him. Miss Emily must have thought that their open courting and his spending the night was a signal that he was ready to settle down. Despite her old maid status, she also maintained the illusion that she was a catch for the right man of her choosing. As for killing Homer, Emily must have felt tremendously dishonored when a common Yankee laborer refused matrimony. She must have considered him socially beneath her, at least according to her late father's reasoning and their own belief of their aristocratic Southern standing in the community. In the end, Miss Emily must have considered Homer a "rat," and her symbolic use of rat poisoning to apparently kill him served two purposes: She would not remain completely alone, and he would reside with her forever; and the community would never know (at least until her death) her secrets--that Miss Emily was a murderer who had been rejected by the Yankee visitor.