Two wrongs don't make a right; A violent criminal suffering a long and unpleasant jail sentence isn't helping their victims.
I'm pretty sure that a violent criminal serving a long and unpleasant jail sentence protects
other innocent people from
becoming victims.
Yeah, but that's because you believe that "violent criminal" is a class of unperson, rather than an action by a real human person.
Most violent crime isn't perpetrated by people who are fundamentally evil or cruel; It's their circumstances, not their personality types, that make them commit crimes.
One (of many) ways to change a person's circumstances in an attempt to ensure that they commit fewer violent crimes in future is to lock them up. It's one of the least efficient, and least effective - violent criminals often commit further violent crimes
while in jail. That their victims might also be criminals, should not mislead us into believing that their crimes while in jail are any less serious than those committed in wider society.
The problem here is that right-wing authoritarians think that they could, if only the bleeding heart liberals would let them, eliminate almost all crime by simply identifying and incarcerating the criminals, leaving the honest, upright and law abiding people free.
But as Solzhenitsyn observed,
The line separating good and evil passes not through states, nor between classes, nor between political parties either -- but right through every human heart -- and through all human hearts.
You cannot incarcerate all the "bad guys", because we are all, given the wrong circumstances, the "bad guys".
The sooner this crazy false dichotomy of good people and evil people dies in a fire, the better. It's one of the most harmful mistakes granted to us by monotheistic religion.
There are a tiny number of genuine psychopaths and other people who either do not resile from evil, or actively seek to be evil. That minuscule population doesn't need jail, it needs secure psychiatric care.
Everyone else just needs to be placed in circumstances where it is preferable
for them to eschew crime as a way of life. The least effective way to achieve this is the threat of incarceration; The second least effective is incarceration itself.