Why does Miss Strangeworth secretly warn people of the possibility of evil in "The Possibility of Evil"?

Miss Strangeworth has been writing her anonymous poison-pen letters for about one year and is getting a lot of enjoyment out of her new hobby. It would seem that she does not understand her true motives for writing these letters or how much anxiety and discord she is causing in her little community. She tells herself that it is her civic duty because she is the oldest surviving member of the town's founding family. This seems like a rationalization. This sweet little old lady has a strong desire to be important. 

Yes, we all crave attention. We want to be important, immortal. We want to do things that will make people exclaim, “Isn’t he wonderful?”

The urge to be outstanding is a fundamental necessity in our lives. All of us, at all times, crave attention. Self-consciousness, even reclusiveness, springs from the desire to be important.   
--Lajos Egri 

Etiam sapientibus cupido gloriae novissima exuitur.
“The thirst for fame is the last thing of all to be laid aside by wise men.”
--Tacitus 

I now perceive an immense omission in my psychology: the deepest principle of human nature is the craving to be appreciated.
--William James 

To be a human being means to possess a feeling of inferiority, which constantly presses towards its own conquest....The greater the feeling of inferiority that has been experienced, the more powerful is the urge for conquest and the more violent the emotional agitation.
--Alfred Adler

It can also be seen from the letters and the people to whom they are addressed that Miss Strangeworth is filled with envy and jealousy. She is an old maid who has never been loved and never had her dream of being a mother fulfilled. She is really a pitiful case, in spite of the fact that she is a busybody and a troublemaker. She has written several letters to Helen Crane hinting that the six-month-old daughter who is the light of her life may be mentally retarded. On the day of the story she is writing a similar letter to Don Crane, Helen's husband. Why pick on these people? Because they are in love and are thrilled to have a new baby. Her letters will spoil things for them. They will be afraid to make love and conceive another child. They will feel less pleased with the baby they already have. And there is probably not a thing wrong with their baby--only a "possibility."

Miss Strangeworth has caused serious problems for Linda Stewart and Dave Harris, a couple of teenagers who are in love and probably intend to get married some day. Why pick on these kids? They love each other and are happy together. Miss Strangeworth has never experienced these things. She has written to Linda's parents suggesting the possibility that the two kids have gone beyond the usual teenage necking and that Linda might get pregnant. 

Some people will try to spoil things for others out of envy and jealousy. It is not to hard to recognize such people by little things they say and little questions they ask. If you feel worse after having talked to them, it is a good sign that you should stay away from them. Oddly enough, the children seem to sense that Miss Strangeworth is not the nice little old lady she appears to be.

Most of the children stood back respectfully as Miss Strangeworth passed, silenced briefly in her presence, and some of the older children greeted her; saying soberly, "Hello, Miss Strangeworth."

In every one of the cases recorded in the story it can be seen that Miss Strangeworth is unconsciously attempting to create discord between others who have relationships she lacks. Eris, the Goddess of Discord, is said to have caused the Trojan War because she was, understandably. not invited to a banquet held for all the other divinities. The ancient Greeks, who derived their gods and goddesses from human characteristics, must have recognized that creating discord was a significant human trait and that it was motivated by bitterness at feeling left out.