There must be a fine line between treating the criminal element in society firmly but fairly. Justice should be dispassionate. Allowing vengeance to enter the system undermines the foundations of a just society. With that in mind, the purpose of incarceration should be for rehabilitation, not for punishment. Even in the most extreme of cases, violent individuals should be kept away from the public for the well-being of all, not because they deserve to be punished for their crimes.
Abusing criminals is telling them they're no longer considered human beings rather than rehabilitating them into society, as should be the goal. This leads to a revolving-door system where newly-freed, dehumanized criminals are unable to adapt to life outside prison and will, in many cases, commit more crimes.