Why did John go back to the South?

In Gary Paulsen's Nightjohn, John is a runaway slave who escapes all the way to the North. However, he enters into the story tied to a horse with scars covering his body. He is immediately put to work, and it isn't until later when he has established himself as a mentor to Sarny that he reveals what brought him back to the South.
In chapter 4, when Mammy is questioning his intentions to teach her daughter, she asks about his scars and wonders if he got them because the masters found out he can read. John replies that he got them when he ran. Mammy assumes he got caught, and John replies, "First time I ran I got clean away. I went north, all the way. I was free." When asked why he came back, he says, "For this . . . To teach reading." This shocks the other slaves, and they grow even more surprised when he continues, saying that he also returned to teach writing. He then explains his capture, saying,

I wasn't going to get caught. I had in mind moving, moving around. Teaching a little here, a little there. But I got slow and they got fast and some crackers caught me in the woods. They were hunting bear, but the dogs came on me instead and I took to a tree and they got me.

Mammy asks why it's important that they know how to write, because "it's just grief for them. Longtime grief." At this, John explains his true reason for returning, saying "[they] have to read and write. We all have to read and write so we can write about this—what they doing to us. It has to be written."Because he was free and safe yet chose to return and risk his life, John's story is a testament to both his love for those who share his suffering and the never-ending persistence of progress.