In "The Monkey's Paw," who do you think was knocking on the door at the end of the story?

The author has told the story in such a masterful way that I cannot help believing it is Herbert standing right on the other side knocking for admission. Like Mr. White, I dread the thought of seeing this formerly happy, playful young man as he must look now. Mrs. White did not see him after he died. He was caught up in the textile machinery where he worked and horribly mangled. Then he was buried and has been decaying for some ten days. Unfortunately, Mr. White did not consider that when he made his second wish. He was reluctant to make that wish at all, but his wife forced it out of him. The author had established that only the owner of the paw could make a wish with it, and Mr. White had become the owner by insisting on paying Sergeant-Major Morris a small sum of money. Mr. White had blurted out the following:

He raised his hand. "I wish my son alive again."

Furthermore, who else could it be out there at that time of night in those weather conditions? The author has not suggested any alternatives. There is one hint that the person knocking at the door could be some stranger lost in these dark, sparsely settled suburbs, who is seeking directions. This hint is given in the following excerpt.

He sat until he was chilled with the cold, glancing occasionally at the figure of the old woman peering through the window. The candle-end, which had burned below the rim of the china candlestick, was throwing pulsating shadows on the ceiling and walls, until, with a flicker larger than the rest, it expired. 

The author V. V. Jacobs wanted to maintain the possibility that the wishes made with the monkey's paw could have all been granted as the result of mere coincidence.

"Morris said the things happened so naturally," said' his father, "that you might if you so wished attribute it to coincidence."

Mrs. White was standing at the bedroom window with a lighted candle (these were still the days before electric lights), and the candle was throwing shadows on the ceiling and walls. A stranger looking for help would have seen that light in all that darkness and would have been sure there was someone at home. That would explain why he continued knocking louder and louder. He was desperate.

Nevertheless, the odds that it was actually the gruesome monster that had been Herbert was standing out there pounding on the door seem so high that it is nearly impossible not to imagine what he must look like and to dread seeing him enter that little house to resume residence with his horrified parents. 

The story resembles one by the Russian writer Leonid Andreyev (1871-1919) titled "Lazarus." It is about the biblical Lazarus who was raised from the dead by Jesus. According to the summary (see reference link below):

Lazarus has just returned home after being dead for three days. Sumptuously dressed, he is surrounded by his sisters Mary and Martha, other relatives, and friends celebrating his resurrection. His three days in the grave have left marks on his body; there is a bluish cast to his fingertips and face, and there are cracked and oozing blisters on his skin. The deterioration of his body has been interrupted, but the restoration, his return to health, is incomplete. His demeanor, too, has changed. He is no longer joyous, carefree, and laughing, as he was before death.