He admits everything.
Sherlock ask why. Old man has said that he wasn’t a good man. In Australia when he didn’t have any luck with finding a gold he begun stealing. He has a nick in Australia: Black Jack from Ballarat. That is the mystery of the word “a rat.” It was Ballarat. One day “Ballarat boys” have stopped a carriage full of gold heading from Ballarat to Melbourne. The driver of carriage was Charles McCarthy. Old Turner was afraid that McCarthy might recognize him and send him to prison and hurt his daughter feelings because he loved her very much.
Sherlock know that Mr Turner will die soon and have let him go. Sherlock have kept the Mystery of Boscombe Pool forever. After three days Mr Turner have died and James have married Miss Turner.