The Rabbis of my tradition made it practically impossible to impose the death penalty. For me, the central issue is truly none of the above. For me the issue of Capital punishment revolves not around how we see the most evil elements of society - but how we perceive ourselves. In the book of Genesis we learn that we are all created in the Image of God. There is a spark of holiness inside every human being.
All life is holy - even that of the most damaged and evil members of our society.
Yes, the murderer has done the same - but the fact that we claim to be a moral society calls us to rise above our desire for vengeance and understand that one act of murder does not make up for another. Killing human beings can never be justified.