> because the purpose of the State, in the first place, is to provide human beings with safety and protection from death
We as an individual can do anything we really want. So if someone wants to kick another person in the head, he could. He will be held legally responsible for kicking the victim.
Ideally, at least this is what the American government is/was supposed to form -- we are the one that gives the government the power. We can totally remove a President if we want. We can dissolve this Union if we want. But that won't happen easily because many of us think about stability, we apply utilitarian views. There is nothing wrong with that.
The issue that you want to raise is the state decides who can kill or not. This is part of civilization. Everyone is supposed to have a role and we determine what people can do and what people cannot do. We want to have police officers who will patrol around with a stick and a pistol. We agree that when the police officer is being attacked he can apply an equivalent counterforce. If someone shoots back, the officer can shoot back. We give the offices the permission to kill another human being because we apply absolutism. We only harm those who harm us directly.
Not every police shooting gets away from legal and moral judgement. Some are found guilty for using excessive force or intentional murdering.
We don't protect people why just educating them. We mobilize them. We arm them, we teach them how to build a healthy body, what to do when they face danger. The world you imagine will not work in any civilization as long as there is a freedom to think. As I said before, the only reason we have desire is because we are allowed to think for ourselves and for other people. We are angry because we have emotions. We become depressed because we have expectations. To protect people who become incapable of staying peaceful and respectful by harming another person physically or mentally (e.g. verbally), we use some form of force on them. And this could be military action (to protect people from invasion) or apprehension. This is true because we value our own life, and we want to be protected.
It is true that some of the military action is morally impermissible in this modern time. We don't need to do a revenge in order to show off our power to scare off our enemies (this was necessary in ancient time), but we shouldn't say that every time we slaughter a human is morally impermissible.
The ethic approach I take is virtue ethics, which looks at the environment, rather than the action or the consequence.
You said what makes us human is that we can decide what not to do and if we allow killing we will go back to animal. I don't see why we should disregard our familiar biological facts. We are animals. Animals have emotions. They can group and hunt. They can think on their own. They may not have science or computer to use, but they are also thinking. In fact, no one can prove or disprove whether physicalism is true or false, whether dualism is true or false. We cannot. We have "the other minds problem". We can't really know what it is like to be a bat (Nagel's paper). We can only speak from our own experience: oh so this is sonic vision and this is how we can see if we had a sonic vision, but that's it.
So what is left with us? We will continue to apply survival of fitness? There was a famous case in England where two sailors were charged to kill this other young sailor boy after the shipped sunk and after they starved for months.
(1) had the boy came up with the idea first and sacrifice himself, is killing that person in the first place wrong?
(2) had the boy asked about this, and the boy agreed, is killing the boy to help the other two more healthy sailors from starvation morally wrong?
Will you be willing to give up your space shuttle ticket to your friend and sacrifice yourself in a zombie crisis? If Russia had dropped an atom bomb years ago, would you think it was okay to destroy Russia like we destroyed Germany by saving the rest of the America? Or do you actually think we can apply peaceful talk?