The Tell-tale Heart by Edgar Allen Poe

True! — nervous — very, very dreadfully nervous I had been and am; 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.

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“Villains!” I shrieked, “dissemble no more! I admit the deed! — tear up the planks! — here, here! — it is the beating of his hideous heart!”