What lesson does "The Possibility of Evil" teach us about human nature?

The main lesson about human nature we should derive from "The Possibility of Evil" is that we should not judge people by the personas they present. Miss Strangeworth seems to be a sweet, kindly little old lady whose main interests in life are doing a little shopping and caring for her rose bushes. But we learn, as does Don Crane, that she has a hidden streak of wickedness. She is a busybody and a troublemaker.

In fact, most people have a good side and a bad side in their characters. This was something Robert Louis Stevenson was dramatizing in his story "The Strange Case of Dr. Jekyll and Mr. Hyde." Dr. Jekyll was a good and altruistic man, but he had a dark side, like every other human being, and he made the mistake of letting it out and allowing it to control him. Nathaniel Hawthorne wrote many tales in which he showed that, although humans may try to appear civilized and benevolent, they all have hidden sinful thoughts, impulses, and desires. Hawthorne shows this in "Young Goodman Brown" and in "The Minister's Black Veil," to name only two of his stories which deal with this theme. Edgar Allan Poe was influential world wide with his stories dealing with the dark side of human nature. Everyone in "The Cask of Amontillado" believes that Montresor and Fortunato are the best of friends--including Fortunato himself. Poe influenced Charles Baudelaire, Guy de Maupassant, Robert Louis Stevenson, Fyodor Dostoyevsky, Arthur Conan Doyle, and many other writers.

Miss Strangeworth is just a feminine version of the hidden evil in human nature. What we should learn is not to be too trusting. People who are too trusting invariably get exploited and hurt. Shakespeare illustrates this in many of his plays. Brutus in Julius Caesar makes the mistake of believing that other people are honorable and truthful like himself. In King Lear, Edmund, a real villain, congratulates himself on being able to deceive his brother and his father so easily. 

A credulous father, and a brother noble,
Whose nature is so far from doing harm
That he suspects none; on whose foolish honesty
My practices ride easy.

According to the pessimistic German philosopher, Arthur Schopenhauer, who had a great influence on many writers including Guy de Maupassant:

It is very important for us to learn early in youth that we are living in a masquerade, otherwise we shall be unable to grasp and get at many things but shall stand before them quite puzzled; and indeed those will stand longest who ex meliore luto finxit praecordia Titan.  ('Whose heart was fashioned by Titan out of better clay,' Juvenal, Satires, XIII, 183).  Such are the favour found by baseness and meanness, the neglect suffered by merit, even by the rarest and greatest, at the hands of men of its branch, the odium incurred by truth and great abilities, the ignorance of scholars in their own branch.  Almost invariably, the genuine article is rejected and the merely spurious sought.  And so young men should be taught that in this masquerade the apples are of wax, the flowers of silk, the fish of cardboard, and that everything is a plaything and a jest.  They should be told that, of two men who are so seriously discussing something, one is giving nothing but spurious articles, while the other is paying for them in counters.