The vast majority of people who commit crimes have been abused, neglected, and traumatized as children. That doesn't absolve them of something they do to someone else, but it does speak to a system where "an eye for an eye" isn't going to result in any sort of social net-positive. Also, a huge proportion of those who are incarcerated are there for low level drug crimes where the only person they hurt is themselves. Then there's the portion that Damian mentions - who may well not even be guilty, but who didn't have the money for proper representation and had to take a plea to avoid an even longer sentence.

In other words, it's really, really not as cut and dried as you'd like to imagine.