How would you write an article about "The Possibility of Evil," or about any other short story, without making it sound like the story.

Edgar Allan Poe has been called "the father of the modern short story." It was Poe who coined the term "single effect" in a review of Nathaniel Hawthorne's Twice-Told Tales. 

A skilful literary artist has constructed a tale. If wise, he had not fashioned his thoughts to accommodate his incidents; but having conceived, with deliberate care, a certain unique or single effect to be wrought out, he then invents such incidents--he then combines such...

Edgar Allan Poe has been called "the father of the modern short story." It was Poe who coined the term "single effect" in a review of Nathaniel Hawthorne's Twice-Told Tales. 

A skilful literary artist has constructed a tale. If wise, he had not fashioned his thoughts to accommodate his incidents; but having conceived, with deliberate care, a certain unique or single effect to be wrought out, he then invents such incidents--he then combines such events as may best aid him in establishing this preconceived effect....In the whole composition there should be no word written, of which the tendency, direct or indirect, is not to the one pre-established design. Graham’s Magazine, May, 1842

The word "effect" refers to the emotional feeling the reader is left with at the end. This is something personal. You could start your article with a description of the feeling you get from reading the story. It may be a "single" effect, but it is not a "simple" effect. It is a combination of shock, dismay, surprise, and other strong feelings, including some pity for this lonely old woman who wants to be important. Then you might analyze how the author leads up to this "single effect" which you personally experience in reading the story. It might begin with the revelation that this sweet little old lady is writing poison-pen letters and end with the destruction of her rose bushes. You might also compare the "single effect" of "The Possibility of Evil" with the "single effect" (you experience) after reading Shirley Jackson's notorious story "The Lottery." Both stories end with strong "single effects," and you would be writing about your own impressions, not simply retelling the stories. 

The "single effect" does not have to come as a shock ending. It can be an effect that is built up throughout the story, or it may come at the very end, as it does in many of O. Henry's short stories. Poe has given us a golden key in analyzing short stories.