Prince Prospero's decision to hold a ball while the citizens of his country fall victim to the most fatal disease they have ever encountered shows just how little he cares about them and the world outside his walls.
The narrator, ironically, describes Prospero as "happy and dauntless and sagacious." He may be happy, but that will only last a short while longer, and he is certainly neither courageous nor perceptive. It is pretty cowardly to hole up in one's castle while one's countrymen die by the thousand, and -- in the end -- Prospero's shrewd plan to stay alive is ineffective.
Further, the amount of money that Prospero spends on a "voluptuous," months-long party could have been put to better use in trying to stop the spread of this disease (or at least to isolate more of the healthy so that they, too, could have a chance at survival). Instead, he spends indiscriminately in order to orchestrate this "masquerade" and the seven rooms, each of a different color, in which it takes place. His willingness to spend money on his own behalf, and for his own enjoyment, rather than using it to assist the citizens of the country further shows his selfishness and lack of concern for the outside world.