If one is familiar with both the works of Edgar Allan Poe and the Ingmar Bergman film "The Seventh Seal," then the identity of the masked figure in "The Masque of the Red Death" is not terribly surprising. In a way, there is no other logical explanation for the figure's appearance and the mystery surrounding it than that it represents that which Prince Prospero and the revelers at his party fear the most: the Red Plague.
Prince Prospero's answer to the plague that is devastating the population outside his castle walls is to seal himself and his friends and followers off from the outside world within those walls. The prince and his revelers mock death (to use a platitudinous phrase) and carry on in a festive atmosphere while outside the walls, people are dying horrible deaths. That mere walls could not keep out a plague is made clear in the story by the appearance of the masked figure, which turns out to be the plague, inside the castle when the prince and the others are ignoring the plight of the outside world and carrying on as without a complaint in the world.
Any contemporary commentary, such as this, obviously benefits from the passage of time and the immunity to such surprises that afflict a culture hardened by today's horror stories and films. To have read "The Masque of the Red Death" at the time of its publication, 1842, would almost certainly have resulted in greater shock value.