I want them locked up, so they don't commit more crimes. Obviously the police don't catch every criminal that commits every crime. So catching them and locking them prevents them from committing more crimes.
Do you have evidence that prison actually reduces future crimes?
Yeah, that's what I suspected. To me, that line of thinking feels short-sighted and/or motivated by revenge rather than rationality.
In the US.
Actually though I think there is a mismatch - there are two groups here, those who made a mistake but are fundamentally good people (they perhaps have a drinking problem or a drug problem, fell into a bad crowd, etc) those should probably not be put in jail, but more a closed treatment center, put on parole, sentenced to community service, etc. The idiots who stole my wheel-caps probably belongs here, as would somebody who killed their spouse because they abused them - it is not okay, but it doesn't make them unredeamably bad people.
Then there are the people who are just scum, the [mum who goes out to drink, leaving her toddler at home](http://www.lohud.com/story/news/crime/2016/05/09/new-rochell...), the gang members, the people who commits rape, etc. Those just needs to be locked up, forever if possible.
It appear to me that both sides of this debate are only focusing on one of the sub groups and are trying to apply rules evenly, without first looking at the criminal. This will absolutely result in injustices, either by throwing a 17 year old in prison for life with out the possibility of parole for [killing her pimp](https://en.wikipedia.org/wiki/Sara_Kruzan) or releasing criminals who will go on to do more criminal acts.