My stance is that if it can be proven 100% that the person committed a premeditated murder then maybe they should be executed. Still iffy on this but besides this no to everything else. We need to sort out problems differently than simply killing people.and as a modern human society its only the right thing to do to try and rehabilitate the lives of other human beings.
Honestly, in a perfect world if I were dictator/Queen, there would be torture chambers dedicated to serial killers/rapists or just plain psychos. In some cases, I don't feel execution is enough. Like, if someone murdered my family, I wouldn't even be able to describe the HELL I'd want them to go through. But since I live on Earth, capital punishment will suffice.