This whole argument is bullshit. The environment shapes human beings, NO SHIT. It still doesn't mean that every human is as strongly affected by this shaping, and if you really believe there are no cases that don't seem to be shaped very much at all, you should probably visit different parts of town once in a while.
You cannot disprove that Rumblood is only mildly affected by his environment. Your sociology or anthropology theories are a result of statistical research that establish trends and predictions for humanity in general or subgroups thereof, but they cannot be applied to individuals just like that.
You should stick to writing fictional stories about how you dropped a guy with a Judo Chop as he was going to throw a punch, they are funnier.
What is so hard to believe about a judo chop? It might be a funny name, but it's just a strike to the jugular, which is quite effective (and dangerous). It's actually pretty easy to do on an unprepared opponent, and also hard to see coming.
A better idea would have been a liver punch (if he was showing his right side), because that's also an almost guaranteed knockout, but much less dangerous than a strike to the throat. Most wannabe brawlers also assume a movie-like stance where they leave their body almost completely unprotected, so that it's easy to put a left hook in under their defense. The problem is mostly that they also turn too much, so that you usually see their left side only. But then you can hit their solar plexus with a left hook, which works almost as good, if they aren't obese or very well trained and you don't hit too high or low.
