The opening paragraph is being narrated in past tense, which is important to note.  It hints that even though the narrator's sentence was death, the narrator might actually survive somehow.  Either that or he is narrating the story from his prison cell.  Regardless, the narrator tells the reader about his trial and sentencing at the hands of the inquisition in the first paragraph.  As he is struggling to come to terms with what his judges are pronouncing, he is rapidly looking around the room.  He's looking for help from any source available, and at one point his eyes fall on seven candles that are burning on a nearby table.  Those candles are what the narrator imagines are angels, and he hopes that they are to there to save him.  He quickly realizes though that the candles are not angels and will not save him. 

And then my vision fell upon the seven tall candles upon the table. At first they wore the aspect of charity, and seemed white and slender angels who would save me; but then, all at once, there came a most deadly nausea over my spirit, and I felt every fibre in my frame thrill as if I had touched the wire of a galvanic battery, while the angel forms became meaningless spectres, with heads of flame, and I saw that from them there would be no help.

What symbolizes angels in the first paragraph of "The Pit and the Pendulum"?

The narrator describes his trial, taking place during the Spanish Inquisition, the room in which the trial takes place, and even the appearance of his judges. He describes the candles burning atop the table in this room, seven candles in all, and he says, at first, that "they wore the aspect of charity, and seemed white slender angels who would save" him. However, the narrator soon realizes that the candles are not good angels at all but "meaningless spectres," with flaming heads, who would offer no help to him whatsoever. They are uncaring, apathetic to his pain, and they offer him no compassion. This transition seems to signal the fact that the narrator will receive no mercy from his persecutors; he is not being watched over by some benevolent creatures but rather by something ghastly and terrible.