That non-linearity applies to sentences for the same type of crime as well. Many of the HN crowd may remember the first chapter of Steven Skiena's Algorithm Design Manual. In a section about logarithms, it showed Federal Sentencing Guidelines for fraud. Stealing $350,000 got a 9 point increase in severity. To get up to 18 points, you needed to steal over $80,000,000! The paragraph ended with:
The moral of logarithmic growth is clear: "If you are gonna do the crime, make it worth the time!"
The reason for this is simple: Sentence length isn't just retribution for the crime. It can be for rehabilitation as well (at least, in other countries). Prison sentences also try to have a deterrent effect. Thieves aren't caught nearly as often as murderers. If the punishment for theft was repayment and a fine, we'd see a lot more theft. It's a similar story with drunk driving. Most of the time, people get away with driving drunk. Nobody gets hurt, nobody gets pulled over. But we want to discourage this behavior since it does kill people. So we make harsh punishments for even a first-time offense.
It's interesting that you bring up the example of Anders Breivik. How would your opinion of him change if, hypothetically, it were later shown that he had a brain tumor? For me, the justification for retribution evaporates. Sure, lock him up forever; he's still dangerous. But if we can trace someone's actions back to causes outside of their control, doesn't that obviate the need for punishment? Then, all we can do is try to rehabilitate them and/or keep them from harming others.
I think this argument applies to the vast majority of people in prison. We don't choose our genes. We don't choose our parents. We don't choose the household or culture we grow up in. We don't choose how much lithium is in our tap water.[1] The vast majority of our personalities and behaviors are shaped by forces completely outside our control, often without our knowledge. We're nowhere close to fully understanding the interactions between genes and environment necessary to diagnose each person's violent behavior, but that's no excuse for exacting retribution. Right now our society is effectively punishing people for having brain tumors. I think focusing on rehabilitation would be much more effective and fair, even for some of the most horrific crimes.
1. Yes, this actually matters: http://www.gwern.net/docs/nootropics/1990-schrauzer.pdf