Death penalty is very difficult issue, from political side, medical, social, philosophical, religious etc.
In case of war, to defend the nation and people, soldiers can shoot and kill the enemy.
However, what if we are not at war?
Does society have a moral right to apply the principle of an eye for an eye, tooth for tooth, life for life, as in the Old Testament?
In the most countries in the world today, answer is simple no.
They do not want to destroy human life, but to rehabilitate the person and allow repentance and new life.
I think that such way is much more humanistic than death penalty practice today in America.
If someone is too dangerous for society, it is clear that we will not let him out.
It's a good point that you are viewing it in many aspects. If we are going to base death penalty in the amount or depth of crime a person does, every individual will be more conscious in the acts they would do. Many are opposing death penalty because of they're advocacy as Pro Life, and for a reason that everyone deserves a second chance. Even if how bad you are, you can still be forgiven. Some would agree to this because of the pain and impact it gives in the society. This kind of punishment really needs more understanding and more studies. So that in the end, no one could say It's unfair!