Prince Prospero sealed himself and his guests in the abbey for more than just one reason, when you really read about it.
The obvious reason is, of course, escaping the Red death. This disease was supposedly ravaging the town and Prince Prospero had the opportunity (due to his rank and position) to localize the disease and move away from it.
The second reason, however, is because he wants to flaunt precisely the fact that he thinks that he is in control, and he invites his courtiers sort of to re-establish his invincible position, entertain them, and show them how much money can actually buy.
Third, he wants to use this escape as an excuse for non-stop irresponsible behavior. While the Red death loomed, people had to be careful not to become sick. Now that the courtiers were away, they felt safe enough to want to tempt fate. They were feasting, and celebrating being better than everyone else.
Yet, when the Red death showed up IN the abbey and surprises everyone, the real weakness of human reality exposes itself.
The Masque of the Red Death, shows us that no one is safe, no matter your social standing. Prince Prospero thinks that because he is behind closed doors he is safe, however he soon learns differently.
Everyone is the town are dying, and they aren't just dying, they are dying in the most horrible and painful way. Prince Prospero thinks that he can seclude himself and some of his wealthy friends inside the walls of his palace. He throws a masquerade ball to prove that he is safe. All of the people come and they are almost throwing their safety in the face of death. Prince Prospero explains that there are horrible things going on outside, but " All these and security were within. Without as the Red Death." Here the prince is showing how cocky he is. He really thinks that his wealth and power will keep him and his friends safe. When death shows up, the prince thinks he is just another person at the party. He is upset that this person would dress up like the death they are trying to avoid and wants this person thrown out of the ball.
"The figure was tall and gaunt, and shrouded from head to foot in the habiliments of the grave. The mask which concealed the usage was made so nearly to resemble the countenance of a stiffened corpse that the closest scrutiny must had had difficulty in detecting the cheat. And yet all this might have been endured if not approved, by the mad revelers around. But the mummer had gone so far as to assume the type of the Red Death."
Once the prince and his friends realize this is the real death, they soon discover that their wealth and social standing is not a guarantee of safety. Prince Prospero understands in the end, that death will come for you no matter who you are.
"And now was acknowledged the presence of the Red Death. He had come like a thief in the night."