these people deserve support and mental care.

And that should be the first option, absolutely. But what do you do when someone refuses to participate in those programs and goes out and keeps harming people? There still needs to be at least the possibility of being put into custody or people just won't take the non-custodial options seriously, especially if they're rich enough to keep paying fines or keep dragging out legal proceedings.

Honestly if your goal is to keep them imprisoned for the rest of their life as a punishment or because you think they’re irredeemable just kill them.

First, any real prison reform would involve radically shorter sentences across the board. In a lot of countries, sentences longer than 2 years are rare and reserved only for the most serious crimes. This is consistent with research suggesting that sometime around that 2 year mark whatever benefits people see from custody plateau or reverse. Ideally, we wouldn't see any sentences longer than that unless the person was an actual threat to the safety of others. More immediately, you can have the most horrible offenders serve more than 2 years while still reducing sentence lengths overall. In some countries you might serve as little as 8 years for murder, for instance. That certainly crosses into purely putative territory (especially considering that most murderers are unlikely to murder again), but I personally don't find that to be terribly unjust.

Second, even if someone appears irredeemably bad or dangerous today, and that would justify long-term imprisonment, it's still a bad idea to kill them. It's impossible to predict how someone might change over 20, 10, or even 5 years, and it's similarly impossible to predict what type of treatment might emerge over the years. And of course there's the ever-present possibility of a wrongful conviction.