I don’t think we gain anything from locking them up forever to be honest. But I am not saying they “can’t” be redeemed - simply that the chance is low and all the time and expense we lose to investing in those individuals (even if only by jailing them) seems like a massive waste when it could go to someone else who hasn’t broken he law. I realize this stance is cruel in a way, but I am simply weighing the trade offs between the different ways we could deal with criminals, and I think it is better for the rest of society to be highly intolerant of violent crimes. For me, violent offense includes any act involving physical violence - including most property crimes. People put their life into obtaining their property - taking it away is the same as taking years of their life away.
Why don't you instead argue for summary execution? Seems like the natural conclusion to your points:
* we gain nothing from locking them up, and it costs us a lot to do this
* redeeming them might not work and if it does it costs us a lot
* we don't want them around
You could try to argue for shipping them to some island, shipping them to another country etc but there's some obvious shortcomings to this such as slave rebellion, other countries punishing through sanctions if you ship your criminals to them etc, so am I missing something or is the natural solution to your position, swift and summary execution?