Reminder that slavery was never outright abolished in the US, the constitution explicitly allows slavery as punishment for a crime which is why private for-profit prisons are a thing in the US.
Can't we do both?
I think the issue is that we're sending people who can be rehabilitated to the same place we're sending the "very bad people" with little to no hope of reintegrating into society
The thing is this is purely cultural conditioning. And every nation has to ask itself what it is willing to sacrifice. In European countries e.g. people sacrifice a few (read 10-20) unlucky victims amongst millions to people that are "too far gone" and exploit the rehabilitation system by being repeat offenders. The US sacrifices extremely large parts of their relative population (over 1 million prisoners) and minorities on very flimsy accounts and puts them away for good, sometimes with no reason. All to prevent repeat offenders and keep them locked away.
So you gotta ask yourself are you willing to completely ruin the lives of a million people (and have probably a greater death toll than a few dozen from that) and not have to trust anybody or are you willing to sacrifice a few innocent people on the grounds of failing to rehabilitate rare "uncurable villain" criminals?