One thing worth considering about the death penalty.
Consider the case of someone killing intentionally or by some mistake, another person. Say a drug deal gone bad, or some similar affair that makes it possible for the guy who did it tyo be considered as having done it intentionally.
This guy now knows that society is out to seek and destroy him. There is no way out, there is nothing to do. Except fight for your life to try to stay alive as long as possible.
You'll die if caught anyhow, so what does a few more deaths mean? Not one thing. In desperation, you'd do anything to try to preserve your life - including trying to kill cops or anyone that get in your way.
Should the criminal ahve thought of it beforehand? Yes. Are people rational all the time? no, and especially not criminals or people with a low education.
To me, punishment is all about "what's best for society". I measure the loss/gains and see the punishment on that basis. I am not sure the death penalty overall score is good enough to warrant it, and that is even without taking into consideration the moral arguments.