This is a great question. The universal conflict in "The Monkey's Paw" is the propensity in all of our hearts to wish for things that we do not have. To put it another way, the moral of the story is paradoxical: beware of what you wish for, because you might get it.
The Whites wish for money to pay off their house, which is something that many people in the world desire today. The Whites get it, but the problem is that they get it at the cost of their son's life. In other words, there are unintended consequences. When this happens, Mrs. White makes another wish: the return of their son. The Whites do not really think of the consequences. What would this look like? How can this happen? When they finally hear a knock on their door, Mr. White realizes that they should not wish for anything else. Hence, his last wish is presumably for his son to return to death. He learned his lesson.
At the beginning of the story, the fakir who "created" the paw made this point.
"An old fakir put a spell on it. He was a very holy man and he wanted to show that fate ruled people’s lives, and that to interfere with fate only caused deep sadness. He put a spell on it so that three separate men could each have three wishes from it."