Theme

The theme of 'The Monkey's Paw' is Fate (and also Greed).
Message: "Don't tamper with fate."
In the story the old man wishes for two hundred pounds, basically distorting and playing with fate. When you tamper with fate your decision often ends up backfiring, as shown in this short story.The consequence of the old man’s wish is that he loses his son.  
 
The message of the story is basically given to us. 
"He wanted to show that fate ruled people's lives, and that those who interfered with it did so to their sorrow."
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This GIF shows Herbert White's death - due to the twisting of fate caused by the monkey paw (and Mr. White): 

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Mr. White says in the story:
 "I don't know what to wish for, and that's a fact...It seems to me I've got all I want."
Mr. White is like Eve in that he has everything he needs and wants but becomes greedy and ends up taking more. Eve is given everything she needs except the fruit growing in garden of Eden and is warned not to, but takes the fruit anyway. Mr. White is also warned of the consequences of using the monkey paw, but does so and suffers the consequences of his greed.

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Mr. White wishing for money (greed). 
The monkey twists in his hand as if a curse or great power has been unleashed and cannot be taken back (twisting fate).
 
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"It moved" he cried, with a glance of disgust at the object as it lay on the floor. "As I wished it twisted in my hands like a snake."
- the twisting of the monkey paw in his hand shows the twisting of fate.