How would you describe the story's atmosphere at the beginning? How does this atmosphere help to emphasize the ending?

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!