Why does Nightjohn come back after he successfully escaped?
Nightjohn told Mammy his story after she asked him about the whip scars on his back. He narrated how he once successfully managed to escape from slavery and lived as a free man. However, he returned because he wanted to teach the other slaves how to read. The venture was important enough for him to risk being subjected to slavery again. Nightjohn believed it was important for slaves to learn how to read and write so that they could preserve their dark history for future generations. He narrated how he was caught and severely punished before being sold back into slavery.
At Waller’s place, he still continued to teach, and this time around his student was Sarny, a young slave girl. Nightjohn taught Sarny the alphabet and how to spell. Sarny’s progress was discovered by Mr. Waller, landing Nightjohn in trouble. Shortly after his gruesome punishment, Nightjohn escaped from Mr. Waller's place, but he returned to continue his clandestine classes. Nightjohn took Sarny as his assistant, and they taught other slaves how to read.