People killing people is one of the better-documented aspects of human history. If it's really a commonly-held idea, it's a delusional one. I think what they mean is 'unethical' rather than 'unnatural'.
It is generally accepted that most people will resist to kill and moral injury is very well documented for example in police who have to use deadly force so the result here is actually surprising. Afghanistan might also be an unusual case by historical standards, because close combat was very rare and the fighting much more impersonal.
Most people will absolutely kill another human if the circumstances call for it. It's in our nature to do so.
What do you base that on?
In one study of WWII soldiers on D-Day, half didn't shoot their weapons because they didn't want to kill someone (my memory of the study is vague). The military had to develop training to make soldiers more likely to kill.
Lots of things are well documented. Famine is too, but it has a big negative impact.
Humans are social creatures that live exclusively in groups; they cannot survive alone psychologically or, usually, physically. Groups are effective when they are at peace; evolutionary psychology, at least, supports not killing each other.
A universal aspect of every human culture is a taboo against killing.
Exactly! It's taboo. It's morally wrong. It's unethical. But does it make it unnatural? It's taboo to talk about sex and what you do on the toilet, two of the most natural and human things there are.