Why is the narrator in jail in the book, Imp of the Perverse?

After explaining his theories in more detail, the narrator says that he has said all he has said thus far as a way of explaining his current condition, which is apparently that he has been jailed. He admits that he had spent months planning the means by which he would commit a certain murder, settling finally on an undetectable poison candle due to his victim's habit of reading in bed at night. The murder went completely undetected and the narrator inherited the victim's estate. He was very proud of the thoughtful execution of his plot, and gained more satisfaction from this than he did from his inherited wealth. For years, everything went well.

After many years, the narrator notices something that he likens to a ringing in the ears, or to getting a piece of a song stuck in one's head. He would catch himself muttering the phrase "I am safe" under his breath. This eventually evolved into "I am safe - I am safe - yes - if I be not fool enough to make open confession!" (pg. 229)

As soon as he says this, he begins to worry that he will confess, a victim of the "perverse" urge he had spoken of earlier. Thinking about it only makes it worse and he begins to panic, running aimlessly through the streets. Finally, he confesses everything and faints.

He has been sentenced to hang.