Very little happens in the early pages of this story. Poe seems to be relying on his powers as a creative writer to hold the reader's attention with descriptions of exteriors and exteriors. In this respect it is a very strange piece of creative writing, even for Poe. It is a relief when the masked figure finally appears because the reader thinks, correctly, that something is finally going to happen. Otherwise, the story is not dramatic. The reader understands almost immediately that this masked figure has come uninvited and has somehow had the supernatural power to get inside the exterior walls and inside the building. The reader understands with the first appearance of Death that the party is over and a thousand people are going to die. The idea of a masquerade recalls Poe's "The Cask of Amontillado," in which Montresor dressed all in black like the black figure in "The Masque of the Red Death," encounters his victim Fortunato in the midst of a similar masked orgy and entices him to his death. "The Cask of Amontillado" is a much better story because it is more dramatic, contains dialogue, and better characterization.
The masqued intruder is the culmination of all things grotesque in Poe's allegorical tale. Once death enters, the reader expects something horrific, certainly, and the Red Death is horrifying. Like the victims of consumption, the Red Death is "besprinkled with death."
For me, it was a sense of fear but also resignation. As the first post says, the masked figure is really quite disgusting, but these things are also frightening. The story up to that point already has a weird and gloomy feel and now it seems like the masked figure is going to make things worse. That's kind of scare.
Given that this is a Poe story, I already expected that the ending would be tragic. So my feeling is also sort of a resigned one. I'm not surprised that this scary figure has shown up. It's partly a feeling of "I knew something bad was going to happen and here it is..."
This is a very subjective question that you are asking, so you might benefit from asking this question on the discussion posting section of this group so you can gain a wide range of different opinions. Really, the answer to this question is going to vary a lot depending on the answerer, but here goes...
When I first read this story (which was a long time ago now), I think I remember being disgusted by the appearnace of the masked visitor and in particular the way he was made to resemble a corpse killed by the Red Death. The description we are given of this figure emphasises how he resembled a dead body, with "habiliments of the grave" and the mask representing a "stiffened corpse," and the features of his face being "besprinkled with the scarlet horror." It is as if Poe is deliberately trying to offend and disgust both us and the revellers at the party, taunting us all with the death that they (and by implication, we) have been unable to escape and dodge. No matter how hard you try, we can never outrun death.