A lot of people think the justice system should be about revenge and not rehabilitation.
Anecdotally, I have spoken with so many people that believe criminals should permanently lose their human rights and never be accepted back into society again regardless of crime committed. It is actually legitimately disturbing how many people froth at the mouth at the idea of a government-run institution that enacts petty revenge on its own citizens. I have had numerous people give me a look of absolute disgust when I tell them that justice systems should rehabilitate people so they can enter society again safely and with helpful resources.
Personally I feel that forcing someone to live out the rest of their life in a steel and concrete cube with no chance of ever being released is more cruel than executing them but I can see how not everyone would agree with that.
If they'd rather punch their own ticket than stay in prison, we should let them, but our legal system just isn't reliable enough for the death penalty to be a good idea. You can always let someone out of prison if new evidence exhonerates them, but if you executed them, all you can do is say 'whoops'
You can tell how humane a society is by how it treats it's criminals, and--importantly--how they treat criminals is how they'll treat their citizens if given half the chance.
Not for no reason does the US lead the western world in state-sponsored brutality and casual administrative cruelty, while northern Europe (who locks their irredeemable criminals away in what Americans derisively refer to as "luxury hotels") leads the world in happiness quotients.
I also think prisoners should be treated humanely, and wonder why people think if we treat criminals like animals they will stop behaving like animals.