Why does Emily want the arsenic?

Emily doesn't tell anyone WHY she wants the arsenic. She avoids the question altogether noting that she just wants arsenic.

"The druggist looked down at her. 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."