How does the initial atmosphere of "The Possibility of Evil" emphasize the ending?

At the beginning of this story, Jackson creates a calm, serene, and idyllic atmosphere. The sun is "shining," for example, the air is "fresh," and the buildings in the town appear "washed" and "bright." This serenity is further reinforced by the strong image of Miss Strangeworth's roses, a symbol of her civic pride.

Through the description of this "fragrant, summer day," Jackson creates an almost-idealized world for the reader in which it seems that nothing can disturb the peace and sense of calm which is depicted in the story's exposition.

As a result, Jackson successfully emphasizes the ending of the story because the reader is completely distracted by the sense of peace that she has created. Moreover, through the characterization of Miss Strangeworth in the opening paragraphs, the reader does not supect that such a seemingly-demure and kind old lady would be capable of committing such horrible crimes.

Jackson, therefore, successfully employs distraction techniques to focus the reader's attention on the idealized town and, more importantly, away from Miss Strangeworth. 

The atmosphere (mood) of the story is one of calm precision.  Miss Strangeworth lives in a neighborhood where she commands the attention and respect of her neighbors.  Everything and everyone is in its place.  The shopkeeper knows what she buys on a regular basis.  Mrs. Crane stops her and looks to her for advice regarding her baby.  Even the teenagers at the post office show her respect.

Miss Strangeworth is a woman of habit, following the same daily routine.  Her house is beautiful. In her home and in her life, it seems everything dare not defy her wishes, and she is appreciated as a good neighbor and pillar of the community.

Shirley Jackson (the author) uses this sense of calm orderliness to distract us.  We excuse the older woman's eccentricities because she we expect it of someone her age, who so lives in the past.  She seems to care for the town in a very personal way.

It is not until we approach the end of the story that contradictions to what we believe about this elderly woman start to surface.  Before we even have time to register what is happening and the significance of these new details, the secret she harbors is out. We are fascinated by the true evil in the story: not of others in the town, but in Miss Strangeworth.  We are shocked by what we learn, and then stunned when she learns of her mutilated roses.

The unexpectedness of the ending works beautifully because Jackson gives us no hint, but sets us up to be blindsided, and that is the beauty of Jackson's short stories.  At the last minute, she pulls the rug out from under our feet—it's wonderfully creepy!

I believe that the story's exposition helps to establish the ending of the story.  Consider the Miss Strangeworth loves the town in which she lives.  She admires nearly every element about it.  She is able to feel a certain amount of pride in the town, a town that she has helped to create.  This helps to underscore the power of the story's ending.  At the end of the story, Miss Strangeworth is able to experience the same level of feeling and emotion about the town.  The ending of the story features Miss Strangeworth experiencing a sense of shock and sadness about the reality in which her roses have been destroyed.  In contrast to the start of the story, where she felt pride at what she created, there is a sense of sadness that has fallen upon her.  The interesting element is that she has helped to create this condition, as well.  Through the writing of her letters, thinking that she has been able to "help" people in the town, Miss Strangeworth has been able to become emotionally impacted by what she has done and what she has created.  The ending of the story is connected to the beginning through what Miss Strangeworth experiences in both.

How does the initial atmosphere in "The Possibility of Evil" emphasize the ending?

At the beginning of the story, everything is peaceful and the atmosphere is celebratory.  

The story begins by describing a lovely walk on a beautiful day.  Miss Strangeworth is described as stepping “daintily” and taking deep breaths on a “fragrant” summer day.

The sun was shining, the air was fresh and clear after the night’s heavy rain, and everything in Miss Strangeworth’s little town looked washed and bright.

The fragrance from the roses foreshadows what happens to them in the end.  They are a symbol of her importance to the community.  This importance gives her the right, in her mind, to meddle in others’ lives.

At the end of the story, the atmosphere does not really change.  It is still cheery, but Miss Strangeworth is upst.  Miss Strangeworth's insult is the product of her own vengence, but she does not see it that we.  She considers the attack on her roses as evil in the world, and does not see all the things she did as evil.