The narrator of "Tell-Tale Heart seems frightened throughout, opening the story with the line "True!—nervous—very dreadfully nervous I had been and am." Even when he thinks up the idea of killing the old man, he says "it haunted me day and night." He suffers, he says, from the "over acuteness of the senses."
His idea of killing the old man revolves around wanting to rid himself of what's been scaring him all these years of working for him: namely, his master's eye, which he says is the eye of a vulture. He says, "Whenever it fell upon me, my blood ran cold."
When he actually kills the old man, he displays more pleasure than fear and says he is glad that his master's eye will no longer haunt him.
However, in the aftermath, his master's death does affect him deeply, and it is the imagined beating of his master's heart that eventually drives him insane.