What is the moral of "The Monkey's Paw"?

The moral The Monkey's Paw is explicitly stated by Sergeant-Major Morris when he tells the Whites that "Fate rules people's lives and those who interfere with fate do so to their sorrow." In other words, people are subject to fate, but trying to take control of one's destiny will lead to worse outcomes than if one just let fate take the lead. We see this when Mr. White's wish for money results in the death of his son. This all raises questions about the role of free will in a person's destiny. Was Herbert's tragic death the inevitable consequence of fate, or were the White's responsible for it as a result of their attempt to meddle with destiny? Can the actions and wishes that we make change the course of our lives? The answer to these questions largely depends on how one interprets this story.
This also leads us to another moral of the story—be careful what you wish for. The first two wishes that Mr. White makes do indeed come true. However, the cost was much higher than anyone would willingly pay if they knew of it in advance. Perhaps the moral here is that we can indeed make wishes and see them accomplished, but there will inevitably be costs and sacrifices that we never can truly foresee.

We do not need a monkey's paw to make wishes. We are free to make wishes at any time. And some of them come true. Unfortunately, it often turns out that the wishes that do come true result in disappointment—or worse. This fact of life seems to be the theme behind the theme of "The Monkey's Paw." We have all had the experience of getting something we wanted and then finding out that we made a mistake in wanting it. An example of a really serious mistake is marrying the wrong person. A less serious mistake is taking the wrong job. Benjamin Franklin wrote the following truth:
All human situations have their inconveniences. We feel those of the present but neither see nor feel those of the future; and hence we often make troublesome changes without amendment, and frequently for the worse.
Wishing is the same as wanting. We all want something we do not have much of the time.
Samuel Johnson wrote a long poem titled "The Vanity of Human Wishes" in which he offers many examples of how people are disappointed by getting something they want. Macbeth desperately wanted to become king, and that turned out to be the worst mistake he ever made. Both Schopenhauer and Emerson speak of a "law of compensation" which dictates that a price must be paid for everything we want.