Ok. All I said was "some." And you said "less than you would think." Less than some is none.
I think there is a visceral revulsion towards the idea, which I find perfectly understandable. Imagine that the fascists had won WWII and enacted a global eugenics program, eliminating anyone with a genetic disease. Later their regime crumbles and we build a kinder, more progressive world - a world with far fewer genetic diseases. Our health infrastructure would be less burdened, as would our economy. We would benefit from a terrible crime, benefits we would absolutely say was not worth the cost.
So it is with capital punishment. I have no idea how strong an effect it has had. It may be very weak, barely measurable, or possibly not measurable at all. But it could be strong. It may turn out that after thousands of years genetic variants tending towards anti-social, violent, or just non-conformist behavior have been weeded out, and as a result society is more stable. I abhor capital punishment (not to mention eugenics) but that possibility remains. The truth is we just don't know, and I'm unsure how knowable it is (DNA from 10,000 years ago being hard to come by).
The thought that we have benefited from 10,000 years of socially proscribed murder is unpleasant to say the least, but I'm not going to say the effect is negligible in the absence of evidence one way or the other. The truth is I have no idea how much of an effect it has had. No one does (AFAIK). I find it hard to imagine that it has had none.