The "Monkey's Paw," is a short story about the age-old warning about greed and wishing. In other words, the story underlines the point about being careful about what you wish, because you might actually get it, but at an unintended cost.
When the Whites wish for their house to be paid off, they actually get it. But they get it at the death of their son. When this happened, Mrs. White does not learn her lesson (nor does Mr. White), they ask for something greater, the resurrection of their son. In this sense, they still do not see that their wishing is dangerous and may have unintended consequences. Finally, when they hear the knock on the door, they wish that it would go way. At this point, they learn the lesson of greed and wishing.
From this perspective, this story is a story with a moral. Be aware of wishing. Curb your desires. This is actually stated in the short story.
‘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.’