Where is the evidence of the benefits of incarceration and rehabilitation of convicted murderers? I've never seen any. I think our society shifted away from capital punishment when social workers and psychiatrists began claiming that no person was irredeemable. They haven't produced proof of their claim yet.
Even the ultra-liberal Star Trek warned against the claim that psychopaths could be reformed:
I would argue that we haven't done much to rehabilitate murderers or criminals. First, let's make sure we're talking about the same thing. Dudes like Bernardo or Clifford Olson are sick and cannot be fixed. They should spend the rest of their lives in prison, and likely will.
Murderers should spend the time they've been sentenced to, and face a difficult time when they are eligible for parole. Not all murders are the same, obviously. There are first-degree, where there was pre-thought and planning, and then murders of passion, as well as manslaughter.
In my opinion, some of these people would never do anything like this again. But, they have literal blood on their hands, and should spend a long time away. But, that doesn't mean they shouldn't have opportunities to work on themselves, like anger management counseling, or get an education.
For them and other types of criminals, there are studies showing that those who get an education are significantly less likely to re-offend. And, considering the cost of incarcerating people, this could be a much cheaper alternative.
I would argue that psychiatrists would not make the claim that every person is redeemable. We know that isn't true, and so do they. Why do you think they recommend some convicts have dangerous offender status like Bernardo. They know they are damaged and would likely hurt people if they were released. They no psychopaths and sociopaths, and know the risks. But, they would also likely agree that not all convicts should be locked up forever.
As for capital punishment, the old saying rings true: It is better that ten guilty persons escape than that one innocent suffer. And, considering the number of people who've had serious convictions overturned (for several reason, unreliable eye witnesses, police/prosecutors ignoring exculpatory evidence, DNA clears them, etc.), I think this still rings true.