The scarlet stains upon the body and especially upon the face of the victim, were the pest ban which shut him out from the aid and from the sympathy of his fellow-men. (1)
Death is isolating. Although one would hope for care and sympathy when one is dying (particularly if it's painful), the Red Death is so fearful that once somebody's caught it he's basically on his own. This loneliness adds to the scariness factor.
And the life of the ebony clock went out with that of the last of the gay. And the flames of the tripods expired. And Darkness and Decay and the Red Death held illimitable dominion over all. (15)
Just like it began with death, the story ends with death. Death was spread everywhere at the beginning of the story, and now it really is: it has broken into the last fortress of life and destroyed it. Now death holds "illimitable" dominion. Put another way, death conquers all.
…and then, after the lapse of sixty minutes, (which embrace three thousand and six hundred seconds of the Time that flies,) there came yet another chiming of the clock. (5)
Poe reminds us again, with a sense of urgency, that "time is flying" – bringing everyone alive closer to their deaths. Poe's division of the hour into seconds makes the passage of time seem faster. It's much harder to ignore it if it's parceled out into smaller pieces, since they go by much more quickly.
It was in this apartment, also, that there stood against the western wall, a gigantic clock of ebony. Its pendulum swung to and fro with a dull, heavy, monotonous clang… (5)
The clock is another clear symbol of death. It's black, imposing, and stands in the black and red room (a.k.a. the death room). It represents the continuous passing of time, which brings death closer every instant.
The seventh apartment was closely shrouded in black velvet tapestries that hung all over the ceiling and down the walls, falling in heavy folds upon a carpet of the same material and hue. But in this chamber only, the color of the windows failed to correspond with the decorations. The panes here were scarlet --a deep blood color. (4)
This room is black and blood red: colors associated with death. It represents death. And it's even more attention-getting than it would be otherwise because it stands out from the other rooms by having windows which don't match the walls. Even in the midst of the Prospero's party, the secluded world fun and safety, death is noticeably present. The interesting thing is that it's there by Prospero's own design…
The "Red Death" had long devastated the country. No pestilence had ever been so fatal, or so hideous. (1)
Poe tells us about the Red Death in the very first line of the story. In other words, we (the readers) are immediately confronted by the threat of death. We're made to feel that it's everywhere – the Red Death is all over the countryside, and has been for a while. And we're also made to feel particularly afraid of death, because the Red Death is "hideous" – it's a horrible way to die.
And now was acknowledged the presence of the Red Death. He had come like a thief in the night. (15)
Now it's revealed that Death itself is actually present at the masquerade. The "thief in the night" bit probably sounds familiar, and that's because it's an allusion to a famous Biblical passage. Poe compares death to Christ at the time of the last judgment. He comes for everyone, when they're least expecting it, and his judgment cannot be avoided.
…when the latter, having attained the extremity of the velvet apartment, turned suddenly and confronted his pursuer. There was a sharp cry --and the dagger dropped gleaming upon the sable carpet, upon which, instantly afterwards, fell prostrate in death the Prince Prospero. (13)
Prospero succumbs to death. His attempts at avoiding it have proven useless. The irony is that he literally charged right into it. And fittingly, he dies right at the threshold of the "death" room.
The figure was tall and gaunt, and shrouded from head to foot in the habiliments of the grave. The mask which concealed the visage was made so nearly to resemble the countenance of a stiffened corpse that the closest scrutiny must have had difficulty in detecting the cheat. And yet all this might have been endured, if not approved, by the mad revellers around. But the mummer had gone so far as to assume the type of the Red Death. (9)
Prospero and his friends have just been reminded about the very thing they were trying to forget: the Red Death. Somebody's had the nerve to dress up as a corpse killed by the disease. Death has infiltrated their secure world. They don't yet know quite how literal that is.
All these and security were within. Without was the "Red Death." (2)
A sharp line is drawn here between a world of life and a world of death. Inside Prospero's abbey there's safety, enjoyment, good times. Outside, there's death. Even though the people inside are supposedly protected from death by being in the abbey, there's something unnerving about the image. Effectively, they're trapped inside, with death all around them.