Well, put it this way..I was a "caged animal" for 13 months, and you would have no issues whatsoever with me around your loved ones, so the logic that the environment is making convicts (the preferred way to label "prisoners" who make it to the state prison level) into monsters is flawed, at least in my case and experience.

I specifically did not say a "large portion", but a significant one, much larger than 1%.

If I remember correctly, approx. 12% of the convicts in the institution I was in (Gulf Correctional Institution[1] in the Florida panhandle) were serving life sentences for murder related convictions.

Does society really want convicted murders roaming freely through it?

[1] http://www.dc.state.fl.us/facilities/region1/150.html

Since most separate state-run institutions for the housing and treatment of mentally ill people were defunded and dismantled, those people are now either left untreated among the public, or forcibly confined (and possibly still untreated) within the jails and prisons system.

So within a typical jail or prison, you will have mentally ill people whose illnesses have caused them to commit crimes, career criminals who have become dangerous and threatening as a competitive advantage, institutional employees who have become domineering, corrupt, and sadistic due to the need to control the previous two groups, and ordinary people who have become stressed and hyper-vigilant due to forced contact with the previous three groups.

Policy in many institutions is also to expressly reinforce racism, creedism, and other natural divisions among the prisoner population as a means of divide-and-conquer control.

As a result, any person spending a significant amount of time in jails or prisons may have problems readjusting to normal society, similar to soldiers returning from a war zone. They are not inherently violent, but the environment is designed to be hostile and stressful for them, and it does influence subsequent behavior in ways that may be unpredictable.