Miss Strangeworth is the descendant of her town's founders, has a beautiful rose garden on Pleasant Street, and acts as if she is full of kindness, caring, and supportive concern for all her neighbors. Everyone likes her and confides in her, as she presents herself as sweet and comforting.
Miss Strangeworth perceives herself as morally purer than all her neighbors. In fact, she believes it is her role to point out even the possibility that an evil might be happening:
The town where she lived had to be kept clean and sweet, but people everywhere were lustful and evil and degraded, and needed to be watched; the world was so large, and there was only one Strangeworth left in it.
The way Miss Strangeworth performs her task of morally policing the town is by sending people anonymous, poison-pen letters that say vicious, hurtful things. It doesn't matter to her whether what she says is true or false. Writing the letters feels good to her and, to her mind, shows she is weeding out immorality and evil.
The irony is that Miss Strangeworth is actually the source of pain, evil, and suffering in the village because of the letters she writes. They are figments of her imagination—"her letters all dealt with the more negotiable stuff of suspicion"—and show her to be filled with the "wickedness" and "dirty" mindedness she is condemning.
Ironically, Miss Strangeworth can't see any of this. She equates the outer beauty of her life, which she tends through her pretty house, roses, china, and silver, with her inner beauty, but in reality she is filled with ugliness. In a final twist of irony, Miss Strangeworth is so blind to herself that she doesn't realize her roses were destroyed because of her own evil:
She began to cry silently for the wickedness of the world when she read the words: LOOK OUT AT WHAT USED TO BE YOUR ROSES.