In “The Tell-Tale Heart,” the old man says, "It is nothing but the wind in the chimney—it is only a mouse crossing the floor" as he tries to reason away his fears by convincing himself that the sounds he heard have logical explanations, such as wind whistling through the chimney or a mouse scurrying across the floor. When he speaks this line, the old man has just been startled awake by a strange noise. He is in complete darkness, and he is afraid.
Most of us have experienced this feeling at some point. Night can be scary for many people. It is a time of increased anxiety and feelings of vulnerability. It is dark and quiet. The darkness brings with it fear of the unknown and the unseen. Nighttime can also make us feel isolated and unprotected. If something bad should happen and everyone around is asleep, who will know or help? As a result of these thoughts and feelings, sometimes we let our imaginations get the better of us. To a troubled, anxious mind, noises from the pipes are knocks on a door, and the wind whistling against a window is an intruder trying to break into the house.
The narrator tells us that he knows the feeling of being frightened at night. He personally understands the terror of waking suddenly to a strange noise and desperately wanting to believe there is a benign, rational explanation for the sound. Even though the narrator personally relates to the feeling of being terrified, he does not sympathize with the old man; he enjoys his suffering.
This tells us that the narrator is cruel and sadistic. He enjoys causing the old man’s fear and anxiety, even though he claims to love him (which is clearly untrue) and knows firsthand how unpleasant anxiousness and fear can be.