The Confessions of Nat Turner, written by himself, is a recounting of Nat Turner's slave revolt. The events described in the book really happened, but Turner proves a somewhat unreliable narrator. He's dictating the contents of the book to his (fictional) lawyer, trying to prevent lessen his conviction. In the end, he's executed along with his various conspirators who are responsible for murdering their masters. Turner takes the opportunity of his confession to portray himself in the most heroic light possible. He seems to believe that he has been destined his entire life to lead this rebellion, explaining that people have always considered and treated him as exceptional.
Growing up, Turner was taught by his parents that he was destined to be a great prophet. This idea was reinforced when everyone around him treated him differently than the other slaves, including his masters. He was taught to read and write, a rare opportunity for a slave. As an adult, Turner took his life purpose seriously. He isolated himself from other people, spending all his free time fasting and praying. He claims to have had a vision of the Holy Spirit one day which revealed to him the information necessary for him to step into his divine appointment of leadership. After this he leads the uprising in which he single-handedly murdered over 50 people, including women and children. The revolt fails after two days when the slaves have no more territory in which to outrun the local militia, but they have wreaked havoc on the communities throughout the Southampton region.
Turner's confession is his final opportunity to claim his superiority. He uses the confession to depict himself as a hero, explaining that it was always his destiny to lead this rebellion. He does not think that he deserves punishment because he was simply fulfilling his duty as charged by the Holy Spirit. Naturally the authorities see things differently. They fail, however, to pick up on Turner's almost certain schizophrenia. He was suffering from paranoid delusions of grandeur which are evident throughout the text. It's doubtful that his mental illness would weight into his sentence even if the authorities had noticed it.