Also, most serial murderers and disturbed enough people to make atrocities huge enough for us wanting them to be killed don't give a shit about dying or not. Executing them is too easy.
Like someone else said, I don't really want revenge. If you want, maybe torture them a little in the same way they tortured someone, but otherwise, no.
For the stats of either death penalty saves or not lives, just cross countries who allow the death penalty to be sentenced and the crime-rate on them. As you can see, the countries with the smaller murder rate owtlaw the death penalty while those who still have it, have worst ones. The death penalty doesn't save lives.
Corelation does not imply causation.