The narrator in Edgar Allan Poe's "The Tell Tale Heart" believes he will get away with murdering the old man because he has planned everything out in his mind, so perfectly. He planned out the death, and after the old man was dead, he very carefully cut up the body. Then he removed floor planks and buried the body underneath. When the police got there, he invited them in, allowed them to search the house, and has already thought of an explanation as to why the old man is not there.
"The old man, I mentioned, was absent in the country. I took my visitors all over the house. I bade them search--search well." (Poe 7)
The narrator then invites the officers to sit down and rest right over the spot where the old man is buried. He thinks he has convinced them that nothing is awry and is quite proud of himself.