How does the mood in "The Possibility of Evil" change from the beginning to end?

Shirley Jackson's "The Possibility of Evil" is similar to her story "The Lottery." In both the mood begins on a tranquil, commonplace, everyday note and gradually becomes more ominous until it is downright sinister at the end. Miss Strangeworth seems like a nice little old lady going about her simple daily routine in a peaceful town where she has lived all her life. She notices some signs of uneasiness and even distress in people she encounters. For example, Mr. Lewis the grocer seems troubled.

Mr. Lewis looked worried, she thought, and for a minute she hesitated, but then she decided that he surely could not be worried over the strawberries. He looked very tired indeed. He was usually so chipper.

Teenage Linda Stewart seems especially troubled. The reader is beginning to wonder what is going on in this little town. It is almost as if clouds are gathering and spreading their shadows all around. Shirley Jackson excels in evoking such darkening mood changes.

Many people seemed disturbed recently, Miss Strangeworth thought. Only yesterday the Stewarts' fifteen-year-old Linda had run crying down her own front walk and all the way to school, not caring who saw her. People around town thought she might have had a fight with the Harris boy, but they showed up together, at the soda shop after school as usual, both of them looking grim and bleak. Trouble at home, people concluded, and sighed over the problems of trying to raise kids right these days.

We will learn later than Miss Strangeworth had sent one of her anonymous letters to Linda's parents hinting that their daughter was having illicit relations with her boyfriend Dave Harris.

Then the mood gets really dark when the reader begins to realize that it is Miss Strangeworth who is creating most of the troubles with her anonymous letters. The author presents the texts of three of these letters and reveals the little old lady's sinister technique of spreading fear, suspicion, and enmity among the people she knows so well. The mood at this point is comparable to that of "The Lottery" when the reader begins to realize that this drawing is something that everybody in the town dreads. In "The Possibility of Evil" it is Miss Strangeworth herself who is the victim. Her letter to Don Crane gets hand-carried to him because she drops it accidentally at the post office. Now he knows the author of the other poison-pen letter regarding his baby which his wife recently received. Miss Strangeworth succeeds in bringing out the evil in this nice, easygoing small-town man. He destroys her prize rose bushes in the middle of the night and leaves her to wonder who might have done it. 

The mood in "The Possibility of Evil" begins in brightness and ends in darkness, just as in "The Lottery." The author's intention in both stories seems to be to illustrate the fact that there really is evil in human nature, and perhaps especially where it is least expected, in small-town America where people seem so neighborly and innocuous. In "The Lottery" the entire town participates in stoning a lone woman to death. In "The Possibility of Evil" it is the lone woman who victimizes an entire town.