The American criminal justice system is way too focused on retribution, with no real efforts toward rehabilitation (a public sentiment that happens to mesh very well together with the privatized prison industry's profit motives). Once you've gotten a murderer off the streets, the public is safe -- killing him or her at that point serves no purpose other than letting us feel like we've "gotten back" at them, or making a governor or other politician look "tough on crime", which are all very immature reasons for wanting to end a human life. Add in inconsistent standards of evidence, biased juries and judges, states that execute people with an IQ of 70, and the lopsided race and gender gaps in death sentencing (for equal crimes, the death sentence is much more likely to be given to non-whites than to whites, and men than women) and it's just a really bad idea.