Capital Punishment is one of those ideas that works better in theory than in practice. In theory, I have no problem with the state killing killers. In practice, the people doing the prosecuting achieve considerable political gain for pushing for the "we're all mad so let's kill somebody" solution instead of actual, you know, justice. Because actually killing the right person may be difficult if not impossible and we can't let that screw up our next election, now can we?
Blatant and well-publicised prosecutorial abuse doesn't even seem like much of a career detriment these days if the new Senator from the Great(ly screwed up) State of Illinois is anything to go by.
There's one exception where I think capital punishment is appropriate and necessary, prosecutorial misconduct be damned: Any legislator caught accepting bribes should be crucified (old-school) in front of their legislative building, and their body left in place to rot as a warning to the others. It gives a whole new meaning to "getting nailed." Or maybe death by impaling. Tough to decide which.