This paragraph is an excellent example of the narrator using his subjective experiences to explain what he sees and hears. He thinks that the "groan" from the old man is in response to an over-active imagination that cannot slow down or see an optimistic response, because that is his own experience. The reader knows that this time, the fear is for a real reason; not a creak in the night, but for a murderer with a specific plan.
I knew what the old man felt, and pitied him, although I chuckled at heart... His fears had been ever since growing upon him. He had been trying to fancy them causeless, but could not.
(Poe, "The Tell-Tale Heart," xroads.virginia.edu)
From this point, the narration is focused on each single moment, from opening the lantern to the final struggle; a suspenseful atmosphere comes from the strain of waiting, waiting for the narrator to act and for the scene to come to a close. Instead, it is dragged out, not boring but almost painful; each moment the narrator expects to be caught, but is in fact entirely in control of the situation. The narrator also expects the old man to be afraid for his life, and the reader becomes afraid in sympathy for the old man.