I found the Babylon 5 episode on death of personality to be instructive. Rather than "barbarically" killing people for crimes, they erase their personalities and give them new, meaningful lives away from their pasts.
That's the worst form of punishment imaginable. It really would be more humane to just kill them, and yet somehow a lot of the characters think it's enlightened and better. For those of you a bit squeamish about taking life, remember that and how it's easier to be bad for goodness sake than good.
I really don't like our modern prison system because it gives a punishment, not restitution to the victims or survivors. Our entire justice system is based on Pavlovian conditioning, not on justice. Of course when you regard justice as Pavlovian, you're going to be against capital punishment. It undermines the purpose of punishment to kill the person you're conditioning.
Personally, I have nothing against capital punishment. I just don't think the option should be taken lightly.