Apologies for my earlier reply, it's been pointed out to me it was rude. It was and I'd like to apologise.
On your point I'm not sure where you get the assertion any human could kill any other in pre-technological society. That appears evidently false to me. How did you come to that assertion?
I would say it is evidently true to me. As stated, humans are fragile. A punch or fall can easily cause brain injury leading to death. Get in an advantageous position on a person and they are going to have a real hard time preventing you strangling them unless they're trained/experienced in hand to hand fighting. On a purely physical level it is not hard to kill a person. This isn't even considering assistance from tools or infection, where a direct kill from fighting isn't required.
The number of people capable of this isn't 100%, sure, but it's closer to 100% than your posited 10 in 1000, 0.1%
On your point I'm not sure where you get the assertion any human could kill any other in pre-technological society. That appears evidently false to me. How did you come to that assertion?