What is Miss Strangeworth trying to accomplish by sending out her letters? Does she succeed?
As an eccentric woman with an overstated sense of personal responsibility toward her townspeople, Mrs. Strangeworth believes that she has the ability to control the town that views her as a sort of matriarch due to the long family history of the woman.
Acting like some form of vigilante, Strangeworth erroneously concludes that, in order to keep the town in check, she should provoke situations by sending anonymous letters to neighbors. These letters put people at odds with each other, provoking the exact behaviors that she claims to want to avert: disdain, hatred, repudiation, lies, and much more. Strangeworth has a twisted mind in the sense that, while she has a good rationale in trying to avert "evil" in town, she is using the most crass methods possible, which lead to the opposite of what should be accomplished.
Mr. Lewis would never have imagined for a minute that his grandson might be lifting petty cash from the store register if he had not had one of Miss Strangeworth's letters.
That is the ironic tragedy. Even when she is caught as the writer of the letters, and the people take vengeance by destroying her garden, she sees that act as the "really" bad thing, and not what she did to the others.
Miss Chandler, the librarian, and Linda Stewart's parents would have gone unsuspectingly ahead with their lives, never aware of possible evil lurking nearby, if Miss Strangeworth had not sent letters opening their eyes
We could argue that she succeeds at causing havoc in the lives of others, whether that is what she was doing in her mind or not. She also succeeds at planting doubt and discord.
Yet, she also wanted to avoid all of that. She viewed her actions as necessary and did not measure the collateral damage that was being caused. In that sense, she did not succeed at averting the evil, but she did succeed at thinking that she may have.
Miss Strangeworth is of the opinion that it is her personal duty as a Strangeworth to watch over her town. She sees evil where there is none; in her paranoid and misguided state, she imagines the worst in everyone. By sending the letters, she is hoping to stop trouble before it begins, or combat the evil deeds committed in her town.
Since the only true evil apparent to the reader is her own, no, Miss Strangeworth does not succeed in making her town better. She makes a lot of people nervous, frightened and sad.
Miss Strangeworth attacks the healthy intelligence of the Crane baby; she causes Linda Stewart's parents to suspect the worst behavior between Linda and Dave, when they have done nothing wrong; and, infers to Mrs. Harper that her husband is being unfaithful. Other letters have driven wedges between people who genuinely care for each other, but Miss S. sees nothing wrong with what she does.
The book says Miss Strangeworth does not deal in facts, but in the possibility of evil. The irony, of course, is that the only evil in the town comes from within herself.