If there was a moral message to this story, perhaps it would read, "Be careful what you wish for." In every case where the Whites make a wish, some awful turn of events occurs to offset the happiness intended by the wish. Perhaps the best illustration of this is the return of the deceased son toward the story's end.
For every item or occurrence they wish for, something negative comes back as a result, even if the wish is granted. The theme that prevails is one that tells the reader to avoid personal greed, as it directly and negatively impacts one's luck in life.
The overall message in this story has more to do with wanting material possessions than it does with luck. Clearly, the White family is much happier, much luckier, at the beginning of the story before they possess the monkey's paw. Once the monkey's paw comes into the White home, you can say that their luck in life ended.
Mr. White is intrigued by the monkey's paw and seems to ignore Sergeant Major Morris's story about the first owner of the magical charm and the fact that the Major wants to get rid of it. Unfortunately, Mr. White does not realize the depth of danger, the tragic consequences that will ensue once he personally wishes on the monkey's paw.
The story of the first owner's third wish for death and his friend's warning does not dissuade him from salivating over the potential material wealth that he can accrue by wishing.
The monkey's paw does not bring luck, or good fortune, just the reverse, it brings misery and misfortune. The paw is cursed with the ability to turn a simple wish into a deeply troubling, tragic moral lesson. Mr. & Mrs. White learn all too well what the real cost of wishing on the monkey's paw is, the price is their only son's life.
After they make the three wishes, needing to use the third wish to return whatever form of their dead son that has risen from the grave back into the cemetery, Mr. & Mrs. White are very, very unlucky, indeed. Not only did they lose their son in exchange for 200 pounds, now they realize the folly of their desire to make a wish at all, but it is too late.