Mrs. Foster is presented as a submissive, patient, anxious woman at the beginning of the story. She could easily be compared with Mary Maloney in Roald Dahl's perfect-crime story "Lamb to the Slaughter." Like Mary Maloney, something comes over Mrs. Foster and she kills her husband--although she does not do it as directly and forcibly as Mary killed her husband Patrick with a frozen leg of lamb. Both women acted on sudden impulse. Mrs. Foster realized that all she had to do was to do nothing. She refrained from going inside the townhouse and told the chauffeur to take her directly to the airport. She knew that her husband was trapped inside the private elevator and would remain imprisoned in it for six weeks, since they were closing up the place for all that time and there wouldn't be a soul to rescue him. Since he was trapped between the second and third floors, deep inside the premises, no one could hear him from outside.

Mrs. Foster changes from a neurotic old woman into a resourceful, independent-minded woman by killing her husband and getting away with it. He has been practicing what is called "passive aggression" on her, so it is appropriate that she kills him with similar passive aggression. Nobody can prove she knew her husband was trapped inside their elevator. The chauffeur could have seen that she didn't go back inside, and he wouldn't have known that she could hear anything through their locked front-door.

Like Mary Maloney in Dahl's better-known story "Lamb to the Slaughter," Mrs. Foster discovers that she can become a good actress under pressure. She returns from an enjoyable stay in Paris and will have to pretend to be horrified when the elevator repairman discovers her husband's body in the elevator. No doubt she will be just as cool as Mary Maloney. She will protest that she thought he was living at his club all this time. She will explain that she hadn't heard from him because he didn't like to write letters. She will have to call in the police, just as Mary Maloney did when she came back from the grocery store and pretended to discover Patrick's body. No doubt Mrs. Foster will tell a credible story at the inquest. Everyone will feel sorry for the poor widow, even though she will inherit the six-story townhouse in Manhattan and all their substantial financial assets. 

Most of all, perhaps, she will get a savage satisfaction out of having taken her revenge on the sadistic man who had tormented her for so many years with his tricks for making her late when, as the author explains,

All her life, Mrs. Foster had had an almost pathological fear of missing a train, a plane, a boat, or even a theatre curtain. In other respects, she was not a particularly nervous woman, but the mere thought of being late on occasions like these would throw her into such a state of nerves that she would begin to twitch.

So Mrs. Foster, like Mary Maloney, changes from a timid, dependent, submissive, chronically anxious wife to a self-reliant, versatile, and independent woman. She does this by a single act, which is illegal and sinful, but certainly understandable and perhaps pardonable. In both cases the women enjoy sweet revenge. Both acted impulsively and rashly, but as Hamlet tells Horatio:

Rashly—
And praised be rashness, for it let us know,
Our indiscretion sometime serves us well
When our deep plots do pall;and that should learn us
There's a divinity that shapes our ends,
Rough-hew them how we will—    (Hamlet, V.2)

There is good and bad in every human being, but sometimes the bad part doesn't come out for a long time.