Actually your assumption is incorrect. According to DOJ statistics, the USA has an average recidivism rate of approximately 60%, and criminals who steal things (robbery, burglary, automobile theft, larceny) range between 72% and 78%.
According to most studies, a statistical majority of criminals actually DO go back to a life of crime after release, and a statistical majority of them get caught and re-incarcerated.
I have a friend who is a Corrections Officer in MD, and he has a MD PCCW. In MD, Corrections officers do not have the power of arrest, and so aren't covered by the Federal Law Enforcement Officers Safety Act. But he has threats on his life and that of his family made on a weekly basis, so he had no difficulty getting his MD permit.
He takes these threats VERY seriously. And since most of the criminals he deals with (even the violent ones) will most likely be released in short time (because MD has such bad overcrowding), he has EVERY reason to believe that some of them may actually attempt to make good on their threats. Luckily, none have yet, but he's not taking any chances. Funny thing is, his wife can't get a permit...
Criminals--especially violent ones--rarely "reform" while incarcerated. Most of them return immediately to their criminal ways upon release. And many of them end up going through their entire life, in and out of the "revolving door" of our "justice" system.
If you don't think released criminals go right back to their evil ways, just ask the families of those four cops in WA if they think Maurice Clemmons had "reformed"...