"Thou shalt not kill" in English, in Exodus 20:13, is most accurately translated from the Hebrew as, "Thou shalt never directly kill an innocent person." Not generally "thou shalt not kill." The Hebrew word used here is thrtzch, which specifically refers to a premeditated, deliberate act by an individual to kill an innocent person.
We humans alone, of all of the inhabitants of the Earth, are sacred.
You are a person, and thus have a right to live.
You have a right to be born.
- Your nature.
- Your origin.
- Your destination.
If life is happenstance, a ghoulish random mixture of amino acids and lightening and comet dust, then you are no more sacred than a dog or tree or any other living thing. But God gave you your nature in a way not happenstance or random. You are not arrogant; you are aware. You have reason. You are free with self-consciousness, able to love in a real way, to think, to pray; to act.
So He gives you your nature, and that makes you sacred.
You are beget from your mother and father, but you are you because of you. God has a one to one relation with you. He Personally invested you with your rationale soul. He is what makes me ME, and you YOU. He knows you because He made you YOU, and not merely the byproduct of your mother and father's genes.
So He is your origin, and that makes you sacred.
You are immortal. Your body will die, but you will never, ever die. Earth will fall away, and all shall pass, but your soul will never die. You will be eternally with Him, or eternally without Him, but your are eternal regardless.
So He is your destination, and that makes you sacred.
I understand why a person would want the death penalty to stop, or want only just wars, but the principle involved is innocence. The death penalty must be just, and kill the guilty. If a person, or a country, is an unjust aggressor, then we have a right to shoot or kill soldiers of an aggressor, and we have a duty to fight wickedness.
So I will close with a constant thought of Dr. Deitrich von Hildebrand: Amoralism is worse than immoralism. Amoralism is indifference. It is lukewarmness. It is vomited out by God. Point out morality. Stand for morals. Stand for truth. Fight relativism and indifference. If you meet amoralism, fight it. Make the person mad if necessary, because when you are indifferent, you are immovable.
God is Truth. Stand for God, and fight amoralism and indifference.