In Chapter 11, John writes home to tell the Creightons that he saw Bill while taking care of rebel prisoners. He told Bill all the news from home, and Bill asks John to tell the Creightons that he didn't fire the bullet that killed Tom.
The reunion between John and Bill shows the importance of family. Bill has had no contact with any of the Creightons since he left and knows that seeing and talking with John will be as close as he will ever come to being with his family. Telling the Creightons that he didn't kill Tom makes them feel better since it shows Bill's love and loyalty to them.