I found the David Friedman quote in “The Pursuit of Justice: Law and Economics of Legal Institutions” and thought that it was worth passing on.
[Suppose you] live in a state where the most severe criminal punishment is life imprisonment. Someone proposes that since armed robbery is a very serious crime, armed robbers should get a life sentence. A constitutional lawyer asks whether that is consistent with the prohibition on cruel and unusual punishment. A legal philosopher asks whether it is just. An economist points out that if the punishment for armed robbery and for armed robbery plus murder are the same, the additional punishment for murder is zero—and asks whether you really want to make it in the interest of robbers to murder their victims (Friedman, “Law's Order: What Economics Has to Do with Law and Why It Matters”).