Maybe going against the grain here, but I'm not on principle opposed to the death penalty. There's lots of strong practical arguments to make, like the fallibility of the court system, but yes, I do believe that there are some crimes (of mass bodily harm) for a which a state could rightfully determine to forfeit someone's right to live. My sole moral objection to the death penalty is that no one should have "end someone's life" as his job description.

However, my version of the world in which the death penalty would be admissible is a lot different from what we have now. In that version, a death sentence would never be executed on first conviction; the verdict would contain a suspended death sentence, more or less. After serving time, a criminal could be put to death only when convicted a second time for a similar offense. I'd also want to see the burden of proof reversed for that punishment: not only must the crime be proven in court (not by plea), but the government must show it provided adequate support and rehabilitation to avoid relapse.

That would be the only world in which I would defend the use of capital punishment. I don't think I'll ever see it happen.