The first wish that Mr. White makes is for 200 pounds.  Unpon making this wish, he initially does not think that it will come true.  The very next day he and his wife receive a call from their son's employer, Maw and Meggins.  They are told that their son had been caught in the machinery at work and was killed; they are then told that they would be getting 200 pounds as compensation for their son's death. 

A little over a week later, Mrs. White wakens in the middle of the night with what she thinks is a brilliant idea -- she is going to make a second wish using the monkey's paw because, as she states, "We've only had one."  To this Mr. White responds, "Wasn't that enough?"  At this point it is clear that he realizes that anything that they wish for using the paw will be gotten only if something bad or evil happens.  Mrs. White ignores her husband and proceeds to wish her son alive again.    Mr. White knows that if the son comes back he will look as he did when he was killed and mutilated in the machinery and since he has, at this point, been dead for ten days, it will not be a pretty sight.   Therefore, he refuses to open the door when they hear the knock and makes his final wish -- probably wishing for their son, Herbert, to be dead again.  So,  he does not answer the door because he believes that his son will be a dead, mutilated body.