Why did Mr.White want his son to die again with his last wish?

The son had already been dead for a while and was
beginning to decompose.  He knew that it was bad to tamper with fate, because wishing
for 200 pounds in money was probably what caused the son to be killed in the
machinery.

Mr. White also probably felt a huge sense of
guilt and dred at the possibility of facing his son after wishing for the money.  Since
the money came from the death compensation from the company the son worked for, the
father would have to explain it to the son.

Finally, Mr.
White was afraid of the curse of the Monkey's Paw and would rather live with the result
of one bad wish than the result of more than one bad wish.