As a result of a deal the Seventh Doctor made with Death, she agreed to transform the Master into the man he would have been if she had never marked him as her champion, on the condition that the Doctor kill him after a decade.
Smith lived for ten years before he was forced to leave his new life. Unlike the Master, John Smith was a kindly doctor with a desire to help others. His favourite dessert was marinated figs with a raspberry coulis, he grew tomatoes, made his own wine, enjoyed theatre, books, and the company of friends, and he was not fond of dogs or people with shifty eyes.
Near the end of his life, his original personality began breaking through, as he became aware of his darker impulses and was troubled by this knowledge. At the end of the crisis, the Doctor asked Death to allow Smith to make the final decision for his future, rather than the Master. When Death revealed the true circumstances of the Master's origin as a result of the Doctor making a deal with her to escape his own guilt, Smith forgave the Doctor, understanding that the adult wasn't to blame for a child's mistake. It is unknown if Smith chose to end his life by killing a man or simply returned to the Master on his own. (AUDIO: Master)