The author said "it is impossible to say how first the idea entered my brain; but once convinced, it haunted me day and night" this supports how he couldn't stop thinking about killing the old man.
"It grew louder-louder-louder! And still the men chatted pleasantly,and smiled. Was it possible they heard not?" The author included this because the man thought the police men heard it too, but it was all in his conscience.
"I admit the deed!-tear up the planks!-here, here,-it is the beating of his hideous heart!" He admitted he had killed the old man, because he thought he was gonna get caught.