In The Possibility of Evil, a 71-year-old woman who has lived in the same town her whole life, and knows everyone and everything in it, is revealed to be the source of cruel and insulting anonymous letters that the townspeople receive, counseling them on their personal business. Mrs. Strangeworth (her name is probably a pun) is oblivious to the hurt she causes, believing instead that she is the town's guardian against wickedness, most of which exists in her overactive imagination.
There are a number of themes that we can articulate from this story.
Evil can happen anywhere, as evidenced by the lack of a name or location for the town, and the embodiment of hypocrisy and viciousness in a seemingly sweet old woman.
Evil can never be eliminated or controlled; in trying to stop wickedness, Ms. Strangeworth begets wickedness, and becomes wicked herself.
Power corrupts; Ms. Strangeworth has power in the form of her reputation and familiarity around town, and she uses this power to control people's lives; it is interesting that she fails to recognize that hiding her identity in the letters may be a sign that what she is doing is corrupt.
One person can make a difference, even if that difference is a profoundly negative one.