Prince Prospero is selfish and cares more about the wealthy members of his kingdom than the general public.
The fact that Prospero builds himself a castle and holes himself up inside of it with the wealthy constituents of his kingdom shows he has no interest in actually taking care of his people. He does not try to stop the plague or protect his kingdom from its devastating social and economic effects. He just flees.
When his dominions were half depopulated, he summoned to his presence a thousand hale and light-hearted friends from among the knights and dames of his court, and with these retired to the deep seclusion of one of his castellated abbeys. This was an extensive and magnificent structure, the creation of the prince's own eccentric yet august taste.
The act of walling himself up inside the abbey and throwing a big party also shows Prospero is delusional. First of all, it is ridiculous to think they will be safe just because no one can get in or leave. Second, throwing a party is a terrible waste of resources when your people are dying. Prospero is in denial.
Prospero’s decorating style is creepy and bizarre. The scariest part is the ebony clock, which chimes to seem like death itself. The revelers stop when they hear it.
But, in spite of these things, it was a gay and magnificent revel. The tastes of the duke were peculiar. He had a fine eye for colors and effects. He disregarded the decora of mere fashion. His plans were bold and fiery, and his conceptions glowed with barbaric lustre.
It turns out that you can't actually just build a spectacular abbey and escape death. Death finds Prospero anyway. He crashes the party in physical form. Prospero's selfishness catches up with him and his guests.