Good questions. I'm going to take them out of order, though.
"What exactly is the judge's role, and who are we to second guess their decision?"
The judge's role is to look at the evidence and the law and assign an appropriate sentence, keeping in mind that the law may state sentencing requirements or recommendations that limit their power. We are not only worthy of second-guessing that decision, but it is meritorious for us to do so in our role as citizens. We don't get to countermand the decision, but if we think that the sentences are consistently unethical in either direction, it's a thing that we, through our democratic processes, can change by altering the aforementioned sentencing requirements in law.
"What is justice, and what, ultimately, is the point of it? How does it relate to courts and the penal system?"
That's the larger question, and I don't have a pithy answer to it. Part of it is ensuring that new crimes are not committed, by this defendant or by others. Inadequate penalties can produce bad results, but massive deterrents aren't perfect either. Some degree of equality in justice is important as well.