How does Tom Sawyer's unselfish treatment toward others demonstrate his maturity?

Over the course of the novel, Tom matures from a young man, who thinks of little outside of himsefl and his own enjoyment, into a young man who comes to learn that there are consequences for your actions.... and that sometimes, those consequences affect others in negative, hurtful ways.

“—and as the doctor fetched the board around and Muff Potter fell, Injun Joe jumped with the knife and—”

“Rebecca Thatcher” [Tom glanced at her face—it was white with terror]—“did you tear—no, look me in the face” [her hands rose in appeal]—“did you tear this book?”
A thought shot like lightning through Tom’s brain. He sprang to his feet and shouted—“I done it!”

"It being strictly a history of a boy, it must stop here; the story could not go much farther without becoming a history of a man."