Replace prisons with compassionate psychiatric hospitals with the purpose of rehabilitating violent criminals who are a danger to the public. People who are not rehabilitatable would still be locked up for the rest of their lives, but they would be treated as humanely as possible. In this case though locking them up wouldn’t be punishment, but rather harm reduction.

There is no utility in punishment. Wanting people who wronged you to suffer isn’t a desire for justice, but a desire for revenge. Dangerous people can be stopped from hurting others without locking them in cages or treating them poorly.