One assumes that to support the death penalty, you would be willing to carry it out yourself (if you wouldn't, then that suggests a fundamental lack of integrity). If that is the case, whether the state sanctions the killing or not at the end of the day you would have to knowingly, in cold blood, kill another human being.

You might be able carry such an action out, I don't know. In all honesty, I would not stoop to killing another human being in such a fashion. After all, what makes the executioner different from the person they're executing? A seal and a squiggle on a bit of paper from some judge who will never have to kill that person themselves? Are you that easily persuaded? Or is it intent - it's OK to kill someone of the intention is to punish them (providing you have the seal and squiggle). Sounds more than a tad uncivilised to me.

Or perhaps the best way forward is not to stoop so low as to kill some of the people who have wronged us, but lock them up for the rest of their lives. That way they are being punished, if there is doubt, mistakes or corruption in the case they won't be dead when this is discovered.