The Purpose of Criminal Law and State Punishment

Sociologists and political scientists of the modern era have largely relied on a few foundational principles in defining a modern state.  Max Weber described a modern state as a political institution which has successfully claimed a monopoly of legitimate physical force- or "violence." While modern states can be described with other monopolies (such as information) or features such as bureacracy, nonetheless the ability to claim the power and legitimacy to use violence is a powerful useful way to describe the relationship between the government and its people. 

Criminal law is not the only expression of such state violence, but it is often the most encountered by the populace on a daily basis. From an arrest, imprisonment and the execution of the death penalty, criminal law is the manifestation of state violence. 

So what makes the use of such violence legitimate? How do we justify the violence that it can impose on people, especially when for most of us there was never explicit consent to the rules imposed? 

In American jurisprudence, views on what legitimizes punishment has shifted, changed and often yo-yos between various different views.  For some, punishment and the violence it imposes can only be legitimate if it provides clear utilitarian benefits such as safety and order.  For others, punishment must conform to principles of justice and retribution, societal views on moral desert must be expressed under criminal laws or it loses legitimacy (and force). 

Of course these views are not binary, and often criminal law vaccilates between serving both such views. 

Imagine however how you would view approaching two different conversations. 

In the first, you are a defense attorney and you need to tell your client that they are about to be imprisoned for 5 years.  Your client for various reasons sincerely believes that they did not do anything wrong morally. (Perhaps they were being prosecuted for vandalizing government property as part of a climate protest, or that they are being prosecuted after exercising their right to self-defense).  How would you explain to them why they will go to prison and what purpose it would serve? 

In the second, you are a prosecutor and you now need to explain to a crime victim why the person who victimized them will not get punished (or punished much less severely).  There has been no dispute as to the facts, but perhaps the person's mental state was found to be less culpable, or that the sentencing guidelines suggested probation rather than jail or prison. What can you tell this person is the reason that the perpetrator is not being punished more severely?

Under both scenarios is there anything said that can make either party feel like the system is justified, fair or legitimate?