Miss Emily actually did not tell the druggist why she wanted the arsenic. The druggist chose to put rats on the package so he would not get in trouble for selling her arsenic. Faukner writes:
The druggist looked down at her [Miss Emily.] She looked back at him, erect, her face like a strained flag. "Why, of course," the druggist said. "If that's what you want. But the law requires you to tell what you are going to use it for."
Miss Emily just stared at him, her head tilted back in order to look him eye for eye, until he looked away and went and got the arsenic and wrapped it up. The Negro delivery boy brought her the package; the druggist didn't come back. When she opened the package at home there was written on the box, under the skull and bones: "For rats."
Miss Emily indicates to the pharmacist that she needs arsenic to kill the rats and pole cats at her home. Otherwise, she would not have been allowed to purchase this poison. Of course, in this time period, no self-respecting gentleman would have questioned a Southern lady's intentions. The reader knows she does have a rat...Homer Baron...who is threatening to ruin her reputation by leaving her after being alone with her in her home. Even as old as Miss Emily was at the time of Homer, a chaperone would have been expected to preserve her "ladylike" status.