There is really never any doubt about the guilt of the mad narrator in Edgar Allan Poe's classic short story, "The Tell-Tale Heart." The narrator explicitly describes his actions, from his planning of the attack a week in advance to the calculating way in which he completes his task. What the narrator refuses to admit is that he is mad--not a murderer.
... but why will you say that I am mad? The disease had sharpened my senses—not destroyed—not dulled them. Above all was the sense of hearing acute. I heard all things in the heaven and in the earth. I heard many things in hell. How, then, am I mad? Hearken! and observe how healthily—how calmly I can tell you the whole story.
The narrator then gives his step-by-step process of the murder process. On the eighth night of stealthily opening the old man's door and peering into his room, the narrator finally takes action. When at last he saw the "vulture eye" open, the narrator attacked.
With a loud yell, I threw open the lantern and leaped into the room. He shrieked once—once only. In an instant I dragged him to the floor, and pulled the heavy bed over him... The old man was dead... Yes, he was stone, stone dead... His eye would trouble me no more.
Afterwards, the narrator dismembered the corpse, placing it under the boards of the floor.
The only argument against the narrator having killed the old man would be if the old man had suffered a heart attack and died during the process--a possibility that even the narrator refused to consider.