In A Rose for Emily what is unclear to the reader when Emily buys rat poison from the druggist?

It is ironic because Emily feels that he is a rat for wanting to walk out on her after making her think he wanted to marry her. She decides to kill this "rat" so what better murder weapon than rat poison. The irony is underscored in the story, because Emily asks the druggist for poison, but when he asks why she needs poison, she stares him down refusing to say anything. The druggist is intimidated and lets her buy the poison with no explanation. He writes down as the reason "For rats." And that is just what Emily used it for.