^^This. There are distinctive differences in criminals. I have a friend that has been in and out of prison and this is the number one complaint of many people. For example, probation requirements following a prison sentence usually require that you have a job. But, companies are often precluded from being able to hire anyone with a criminal record (even non-violent records) due to (primarily) insurance costs. So, these millions they send to prison for non-violent crimes end up worse off than they were before prison and in the case of drug related crimes, recidivism rates rise.
I also have a friend that is a guard at a maximum security prison (very violent criminals) and from the things he describes the criminals there are complete animals. Some of the things he says he sees and hears are mind boggling that in such a highly technical society and in this country where standards of living are much higher than most, it is baffling how people could turn out to be as barbaric and animalistic as the criminals there.
It is this second class of criminals that need to be locked up and as far as I'm concerned they could lock them up in one big area alla "Escape from New York" and just let them go animal on one another rather than citizens having to risk their lives guarding them.