Study law with the Jesuits in Rome and you are not only required to take philosophy of law but theology of law as well. Ulpian's definition of justice is drilled into you (in Latin)
constants et perpetua voluntas ius suum cuique tribuendi.
The constant and perpetual will to render to each his due.
What then is the purpose of punishment?
If you look at the definition of justice it ends with the word tribuendi, to render or to give. Punishment is about retribution. Retribution is a word that has gotten a bad rap. It means simply to give back.
Exodus 21:24 - eye for eye, tooth for tooth, hand for hand, foot for foot
It sounds harsh but it was actually intended to set the limit, drawing the line between retribution and vengeance. In retribution you require no more of a person than what was taken. If a person stole your horse you were entitled to a horse. You were not entitled to kill them. Retribution is about restoration. To exact more than was taken is not retribution but vengeance.
The second purpose of punishment is the conversion of the offender. Our English language understands this. In the old day here would you go to do your penance?— a penitentiary. Unlike our modern facilities, they were often associated with monasteries intended to be places of prayer and conversion not containment for its own sake.
In a just system, the second purpose of punishment is to cause a change of heart in the offender so that the offense will not be repeated. In the Church we refer to them a medicinal penalties. We see even the worst criminal as a human being in need of healing.
Restoration and transformation/healing these are the purposes of punishment in a truly just, justice system.
In our American system we grant certain judges lifetime appointments to insulate them from the mob that cries out for vengeance, so that they may render true justice.
In a few hours Judge James Spencer will hand down the sentence in the case of former governor Bob McDonnell. It is my hope and prayer that Judge Spencer will rise above the clamor and show the world true justice. And perhaps even justice tempered with mercy.