When we read this excellent short story it is hard not to escape the fact that one of the moods that Poe so excellently created is terror and fear. This is a story that you would be wise not to read late at night if you were by yourself and in the dark! Let us explore how he establishes this mood by analysing a few key images that he creates.
I think one of the images that certainly creates this mood of horror in me is when the unnamed narrator is in the old man's room at night, silent and still, waiting to kill him, but then he hears the old man utter a groan of "mortal terror":
Presently I heard a slight groan, and I knew it was the groan of mortal terror. It was not a groan of pain or of grief--oh, no!--it was the low stifled sound that arises from the bottom of the soul when overcharged with awe. I knew the sound well. Many a night, just at midnight, when all the world slept, it has welled up from my own bosom, deepening, with its dreadful echo, the terrors that distracted me. I say I knew it well. I knew what the old man felt, and pitied him, although I chuckled at heart. I knew that he had been lying awake ever since the first slight noise, when he had turned in the bed. His fears had been ever since growing upon him.
Note the description of the old man and the way that he is left prey to his own fears, which in many ways is worse than being aware of what the threat is, as our fears can "grow upon us." Also note the way that the narrator, although he pities the old man, also "chuckles at heart," making his act and the vision of him waiting silently in the old man's room whilst the old man is terrified that much more sinister and terrible.
Poe therefore creates a mood of horror and fear by deliberately playing on our own preconceptions and terrors, presenting us with a man lying awake in his bed knowing that someone or something is in the same room as him, waiting to kill him.