For a really good but brief read, check out C.S. Lewis’ classic The Humanitarian Theory of Punishment, in which he discusses why it is inhumane to NOT punish evil-doers with just punishments.  In a nutshell, he says the following:

  • Punishment is not immoral, barbarous, or revengeful by definition
  • While we should consider the deterrent effect of our reactions to crimes, and while we should think about "curing" the malefactor, neither of these are the primary reason why punishment is or should be dealt out.
  • If we remove just deserts, i.e. punishment, from the equation, we remove justice entirely
  • Replacing punishment with mandatory cures results in (1) indeterminate sentences ("is the patient cured yet?") and (2) it removes all possibility of restitution and purging of guilt

If crime is only a disease which needs cure, not sin which deserves
punishment, it cannot be pardoned. How can you pardon a man for having
a gumboil or a club foot? But the Humanitarian theory wants simply to
abolish Justice and substitute Mercy for it. This means that you start
being ‘kind’ to people before you have considered their rights, and
then force upon them supposed kindnesses which no one but you will
recognize as kindnesses and which the recipient will feel as abominable
cruelties. You have overshot the mark. Mercy, detached from Justice,
grows unmerciful. That is the important paradox.