It's also about keeping violent people off the streets.
This guy killed someone. He mentions it so briefly it's as though he thinks it's a minor detail. That person he killed was a real human. They were someone's kid, someone's sibling, someone's parent perhaps, someone's friend.
He also seems to behave poorly in prison, still, after 30 years. He flippantly notes a "riot" as though it was something that happened to him. He brags about his abs. I'm not a psychologist but this guy reads like a stock standard psychopath. https://en.wikipedia.org/wiki/Psychopathy
For many people (including myself), it's not about rehab, it's not about penance, it's not about vengeance, it's about keeping someone like this away from everyone else.