Presumably, Emily has poisoned Homer in order to keep him from leaving her. By the time she meets Homer, her father has passed and left her alone after having driven off any potential suitors in her past. Since she is getting old and is unlikely to find another beau who will marry her, she becomes desperate when she realizes Homer (who "wasn't the marrying type" because, as he said, "he liked men"), was going to leave.
Terrified of being forever alone, she poisons Homer in an attempt to keep him. He is in her mind, much like a cherished rose given to a young woman by her beloved and pressed between the pages of a secret book, preserved forever in her rose-colored bridal suite. And just as a woman might revisit her withered and dried rose from time to time in order to recapture the moment of its newness, Emily visits her dried "rose" (Homer) and lies with him, remembering a dream that she cannot let die, but that could never come true--a dream of a life with Homer. Her murdering Homer was an act of desperation--and an attempt to avoid her inevitable loneliness.