First, because Humans are not prescient. Nobody knows what someone will or won't do in the Future.
And Second, because it's not as easy a thing as you might think.
..snip...
Killing someone changes you, even when justified or in self defense. Ask any cop or soldier who's ever had to kill someone and they'll tell you, they do not just go home and sleep well. They may understand logically that it was something that had to be done, but it stays with you and weighs upon you, and never really completely goes away...ever.
Taking a life is a monumental thing. You're not just killing them in the moment, you are killing them in the future. Everything they would have been is now gone. And no matter how unemotional a person may be or seem, unless one is a true psychopath, it's virtually impossible to not feel an involuntary empathy for the person they just killed. You see it in the dying persons eyes. You see the end of thought, personality, character, being...LIFE...in those eyes. And in that moment, justified or not, necessary or not, self defense or not, it is an extremely hard thing to see, and impossible to forget. Once the adrenaline is gone, once the rage or fear has faded, the true impact is always felt and cannot be escaped.
There is always a price.
It depends on the person.
Of the people drawn to police work or soldiering, some percentage of them do not posess the "killing people is wrong" gene. They do not have a problem taking a life. Which is some of the reason they were drawn to that work.
So it's not fully true that everybody pays that emotional price.
Certainly, people who empathize with others are going to have this stress and in fact are unlikely to be willing to follow through for the same reasons you cite.
But once upon a time, we used to be able to go over and raid the other tribe and were celebrated as heroes by our village when we returned. We were not haunted by nightmares of the faces of the men we slew.