The denouement of "The Possibility of Evil" is what we would call a climactic ending. The ending is climactic because it is unmistakably life-changing in nature. Miss Strangeworth's habits are no longer a secret; someone is privy to her actions. For the woman who has never contemplated the "possibility of evil" in her life, the revelation that her position has been reversed is terrifying. The hunter has become the hunted. The ending is climactic because it is stark, shocking, and ironic: Miss Strangeworth's life is forever changed and impending events are beyond her ability to control.
We are told that Miss Strangeworth sent out three letters during her last outing. The envelopes were pink (to Don Crane), green (to Mrs. Harper), and blue (to Mrs. Foster). The Harris boy retrieved what appeared to be a pink envelope and presumably delivered it to Don Crane. However, the envelope (and letter) that returns to Miss Strangeworth is green in color.
We know that the green envelope was sent to Mrs. Harper. Here's the quote that describes what Miss Strangeworth wrote to Mrs. Harper:
She selected a green sheet this time and wrote quickly: Have you found out yet what they were all laughing about after you left the bridge club on Thursday? Or is the wife really the last one to know?
The phrase "the wife is usually the last to know" refers to how a woman often discovers her husband's affair after everyone else. The quote above is intriguing because of how it characterizes Miss Strangeworth as an omniscient purveyor of justice in her community. Yet, the climactic ending turns this characterization on its head. Instead of being the powerful, righteous matriarch presiding over a wayward community, Miss Strangeworth becomes a terrified old lady at the end of the story.
Someone else knows about her secret life. The reader is left to contemplate the possible identities of the one who has discovered Miss Strangeworth's secret. Is it Don Crane, since the Harris boy delivered his letter (and very likely revealed that Miss Strangeworth sent it)? Or is it Mrs. Harper, the one Miss Strangeworth believes is the last to know about her husband's affair? After all, the letter that was returned to Miss Strangeworth was green. Or, even more intriguing, can it be that more than one person knows about Miss Strangeworth's secret?
Whatever the answer is, the ending is a climactic one. Miss Strangeworth's life is forever changed and not in a very pleasant way.