The exact reason due to which the narrator kills the old man in Edgar Allan Poe’s The Tell-Tale Heart is not very well understood or clear. The narrator himself is not sure what invoked the idea of killing the old man.
It is impossible to say how first the idea entered my brain; but once conceived, it haunted me day and night.
Since there is no exact reason behind the murder, we are bound to think that, perhaps, the narrator is insane or some kind of a psycho-killer. The narrator, however, insists in the first few lines that he is not mad since he carefully planned the murder with such precision. But, of course, this explanation is not very convincing.
TRUE!—NERVOUS—VERY, very dreadfully nervous I had been and am; but why will you say that I am mad?
The narrator says that he had no real issue with the old man and, in fact, he loved him. He also mentions that the old man never did anything wrong with him and never ever insulted him. The old man was quite wealthy, but the narrator said that he was not interested in his wealth also.
He had never wronged me. He had never given me insult. For his gold I had no desire.
So why would the narrator kill him? The narrator does point out that he did not like the old man’s eyes. He got uncomfortable and nervous when he saw his pale, blue eyes like that of a vulture. He claims that he wanted to just get rid of those eyes.
I think it was his eye! yes, it was this! He had the eye of a vulture—a pale blue eye, with a film over it.
A reason like this to murder someone is not justified. Only someone really mad would think like that. The narrator kills the old man very brutally. In fact, just note the way the narrator handles everything with the policemen after he murders the old man. All this also give a hint that the narrator is an unbalanced, impatient and insane person.
The narrator repeatedly tries to explain that an insane person cannot do such a calculated murder and this makes him absolutely normal. But we are forced to think that the narrator is, indeed, crazy. So, we can say that the narrator killed the old man because he is mad. The poor old man was misfortunate to have become the target of the narrator’s madness.
Because of his "vulture like" eye, And he was just plain out crazy