The whole poison-purchasing scene is very interesting and adds to the impact of her action. Emily is determined to buy poison and let the pharmacist assume it is to kill rats. While he is adamant about knowing the truth, Emily is not interested in sharing the details of her plans with him. I want some poison," she said to the druggist. She was over thirty then, still a slight woman, though thinner than usual, with cold, haughty black eyes in a face the flesh of which was strained across the temples and about the eyesockets as you imagine a lighthouse-keeper's face ought to look. "I want some poison," she said.

Yes, Miss Emily. What kind? For rats and such? I'd recom -- " want the best you have. I don't care what kind."

The druggist named several. "They'll kill anything up to an elephant. But what you want is -- "

Arsenic," Miss Emily said. "Is that a good one?"

Is... arsenic? Yes, ma'am. But what you want -- " want arsenic."

Faulkner, 144-145)

Emily wanted the best kind because she didn't want an inferior poison to foil her plan. This shows her commitment to her dark ideas. She must have been contemplating murder for sometime. Purchasing rat poison was the best choice because it was an ordinary item for most people and this would have never given anyone a hint as to what she was about to do.

Poison always signifies death and decay. This was also in tandem with the whole plot and the character of Emily. Emily was a symbol of decay herself since she had given up on life and...

Her father also played an important role in turning her into a symbol of early death. "Miss Emily Grierson, a member of one of the oldest families in Jefferson, lives alone in her frame house, afraid to go out after her father dies and her sweetheart deserts her. Emily's attachment to the will of the father -- it is said that he had driven all the young men away -- has stunted her growth." (Brooks et al. 37)
Emily symbolized decay as we notice that her house was situated at a dead end. She was dead herself since she stayed away from everyone and hardly ever socialized. The one man in her life was her father who died in mysterious circumstances and whose body, Emily refused to give up for burial. She had become obsessed with death and decay and thus chose poison as her obvious weapon for bringing an end to her already half-dead life and to that of her lover's. While suicide was expected of her since she was such a recluse but people never imagined that she was capable of murder too. Purchasing poison in fact foreshadows the bleak ending since near the end we also discover that she had been sleeping with Homer's decayed body for many years before she died.