In "The Monkeys Paw," why does Mr. White not want to answer the door when he knows it's Herbert?

Mr. White has figured out the whatever is on the other side of the door should rightfully be sent back to the grave.  By the time that Mr. White makes the first wish for 200 pounds and makes the last wish for Herbert, or whatever is behind the door to return to the grave, he has figured out that the monkey's paw is an instrument of dark magic.  

Mr. White realizes that whatever form Herbert is in on the other side of the door, it does not belong out of the grave.  The man was crushed by a machine, he is terrified at what the wish might have done to reconstruct a form of Herbert.

Although Mr. White had given in to Mrs. White's pleading to make a second wish on the monkey's paw to bring Herbert back, when he does not return immediately, Mr. White becomes suspicious.  He realizes that with his son dead for ten days, he cannot imagine what kind of monster is really at the door.

"For God's sake, don't let it in," cried the old man trembling. (Jacobs)

  "You're afraid of your own son," she cried, struggling. "Let me go. I'm coming, Herbert; I'm coming."   (Jacobs)