This is all about nature. Not just human nature, but nature itself, of which we are a part.
Urwumpe, it's one thing to want to improve yourself, but that means accomplishing more, not trying to overcome nature. What I mean by that is - changing your diet, improving your physical form, learning something new, being less lazy/working harder, getting a better routine going (could be anything, work related, exercise, morning/sleep, etc), caring more about those who are important to you/care about you, etc.
However, to sit there and claim that "violence is an abberation and must be eradicated", or that competition is bad, or that aggression is bad, etc, THAT is a problem.
The people we here in the states call liberals (not sure what you guys in Europe would call them, but I know you use "liberal" different than we do) are hippies, touchy-feely types who claim to love nature, but in fact only like trees but hate everything else about nature (because it's violent, cold, harsh and unforgiving, and decidedly "unfair"). They think they somehow know better than the very thing that gave them an existance in the first place.
That isn't to say that one should be out of control and hyper-violent. But it IS to say that violence and aggression are NOT the evil thing they are made out to be, and we as a society NEED to be more accepting of it. It's actually no wonder that crime is so bad today, we have no outlet, and video games just don't cut it. We need to bring back "the games" (just no animal sacrifices this time). We've taken away all outlets and ability to respond in appropriate manner, but left (even increased) the stress and pressures. It's a bad situation.
Couple that with an out of control population level (and growth), and you're going to have a larger absolute number of people too weak to take the pressures, and they snap. Columbine is the perfect example.
That was a case of weak individuals, horrible parenting, and a social system gone completely to hell. You can NOT defend your honor anymore. You have to sit and take anything anybody says or even does to you, in many cases, even RETURNING a strike will get you in trouble (meaning, taking the second swing, not the first). But nothing gets done about the pressures applied from peers, responsibilities, parents, the media, etc, etc, etc. And for kids, who haven't acquired all the tools they need yet, and are at the same time dealing with the CRUELEST peer group on the face of the planet (other kids), if they are trapped by society and have no guidance at home, the slightest weakness can make them snap.
I have some experience here. I was an outcast. I know exactly what it's like. But I didn't haul off and gun down my classmates either. Why? 2 reasons, I'm stronger (internally) than those kids, and perhaps most importantly, I had a release - I punched the elemntary school principle in the face, laid out one of the kids harassing me twice, sent another to the hospital to get lead removed from his skull.... lol Yeah, sounds bad, but all I wanted was to be left alone, and they just couldn't do it. Also these events were strung out over several years, so it wasn't like I just fliped out and went on a rampage. It takes me a while to build up a head of steam, but every pressure cooker needs a releif valve. Take that away, and don't be surprised if a few of the containers start exploding here and there.
And it all stems from the same source - all of it. Our love for violent entertainment (games/movies/tv shows/sports/etc), rough play/horsing around, competitive behavior, aggression, etc, it's ALL from an ingrained competitive/aggressive nature. It's part of being a social mammal. We compete against our predators and our prey, and against each other for status and rank. The alpha-male gets the most foot, the most women, and the most offspring, thus we all want to be there. Females are tuned to this too, as the alpha-male will likely be the stronges/smartest/most dynamic/etc so he will be able to provide the most, and most importantly, his genes will have the greatest chance of survival, and of being attractive to successive generations. We are nothing more than vessels for our genes really.
If you remove competitive tendencies from humanity, it will just lay down and die.
This is something that's rather obvious to those who understand, and apparently Joss Whedon is one of those people. Ignore that this is from sci-fi, and ignore the ultra-violent backlash (may or may not happen, but it makes for great story). It's a moral lesson about tampering with nature and being so arrogant as to think that you know better and can actually "improve" it.