Consider the way in which the Whites first respond to the sergeant major's announcement of his possession of some real Indian magic. When he introduces the monkey's paw and then tells the Whites that it is "just a bit of what you might call magic," the text shows the interest of the Whites by describing how attentive they were: "His three listeners leaned forward eagerly." However, the way in which their first wish is actually granted seems to point towards their understanding of the lesson that the monkey's paw was created to teach. Note the response of Mr. White to the $200 he is given as compensation for his son's death:
Unconscious of his wife's shriek, the old man smiled faintly, put out his hands like a sightless man, and dropped, a senseless heap, to the floor.
Clearly, this strongly suggests that Mr. White has learned that "fate ruled people's lives, and that those who interfered with it did so to their sorrow." This is supported by the way in which he uses the final wish to cancel the second. His wife, on the other hand, clearly has not learnt this lesson, as her determination to get her husband to use the monkey's paw to restore their son to them shows.
In addition to what has been posted as an answer to the question, I would like to add the reaction of Herbert, the son. He was always doubtful regarding the powers of the paw. In the first scene, he sarcatically remarks that his father could wish to become an emperor, and then he shot both the parents with the same arrow.
"..........said Herbert with pretended horror."Why, we're going to be rich, and famous and happy. Wish to be an emperor, father, to begin with; then you can't be henpecked."
Moreover, one cannot forget that the next day, in the clear bright rays of the sun streaming into the room, Herbert felt a visible change in the room, for the depression and fear had somewhat dissipated.
"In the brightness of the wintry sun next morning as it streamed over the breakfast table Herbert laughed at his fears. There was an air of prosaic wholesomeness about the room which it had lacked on the previous night......."
Remember, even the mother felt that sergeants like Major Morris were always exaggerating and his story about the monkey's paw was as unseeming as were the other stories of his escapades in India.
" ........ nor prevent her from referring somewhat shortly to retired sergeant majors of bibulous habits when she found that the post brought a tailor's bill."