The author of the monkeys paw write this story to show that fate ruled people's lives and that those who messed with it would regret it.
In the story the White family didn't think the tales of the monkeys paw was true so they tested it and wished for two hundred pounds; nothing happened so they just waited, the next day their son went to work, a few hours later his boss visits the mother and father who wished for the two hundred pounds and told them that their son died in machinery and gave them two hundred pounds, the two hundred pounds that they wished for.
When the mother had grief about her son dying she wished to have her son alive again; but the only problem was that their son, wasn't their son anymore, he had been caught in the machinery and torn apart by it. The father knew this and that's why he tried to stop her from wishing him alive again. With the final wish they had he wished for the son to be gone.(RIP)