I can strongly recommend The Faithful Executioner: Life and Death, Honor and Shame in the Turbulent Sixteenth Century by Harrington. It is based on the diaries of a Franz Schmidt, an executioner and therefore also torturer in Nuremberg. He certainly exhibited no signs of going insane, although his worldview evolved through his life.

At that time, in medieval and early Renaissance Germany, executioners and torturers were outcasts. At the same time, Schmidt was permitted to practice basic (emergency room) medicine. For this he received respect. Through the care he devoted to his work and his concern for always maintaining a professional, respectful image, he was elevated from the "untouchable" caste from which he began his life, and was able to save his sons from being forced into their father's profession, which was typical.

There is more discussion of Schmidt and other sources here: https://www.reddit.com/r/AskHistorians/comments/48kfx5/historiography_of_early_modern_german_executioners/