Obsession
"It is impossible to say how first the idea entered my brain; but once conceived, it haunted me day and night."
The narrator's entire basis for killing the old man is because of this strange, irrational fixation on his eye. The narrator even fixes this idea of an "Evil Eye" in his head and states that "Whenever it fell upon me, my blood ran cold." It is for this reason, and this reason alone that he decides to take the old man's life. As the narrator puts it, "Passion there was none. He had never wronged me. He had never given me insult". We must then wonder why he chooses the eye as the sole fixation of his murderous intent, which is discussed in detail here. But even without knowing why, we can clearly see his obsession over it, and therefore its power over him. His fear of the eye eventually leads to his obsession over it, and his eventual insanity in his decision to murder the old man, in order to finally get rid of it. We can see the lengths that his fear and obsession drive him to as he recounts his inability to sleep, sitting up "night after night", and the enormous effort and time in which every night for eight consecutive nights, over the course of more than an hour, he would peer in on the old man while he slept and shine a "thin ray upon the vulture eye".