Releasing a violent prisoner, or keeping a harmless one incarcerated?

How violent, how preventably violent, how harmless, how incarcerated, how long incarcerated? For any specific case with these agreed-upon, I am confident a supermajority would agree.

That people do NOT agree suggest to me that it's hardly a trivial question...

That people don't agree suggests one side is comparing releasing a serial killer to incarcerating a drifter in jail a short while, and the other side is comparing releasing a middle-aged man who in a fit of passion struck his adulterous wife to incarcerating Ghandi for the term of his natural life. More generally, they are deciding based on one specific example they have strongly available to them.

In the state you phrased it, that question is about as answerable as "how long is a piece of string?".