Nature definately plays a role, but I think how we express our violent tendancies is based on nurture.
We are becomming more violent because violence is both suppressed and encouraged by our society.
I was watching CNN Anger in America yesterday. They had a guy on who was trying to get guns off the street in his community. He expressed it perfectly, he said the biggest guy with the biggest gun is the most important in the neighborhood. Violence results in elevated social status, this is a very powerful motivator, ask any anthropologist. Add to this our myths (Stories designed to transmit culture, in the US conveyed through movies, TV and video games) involve violent people doing violent things, without the tempering influences of the past that come from Religeon and more traditional morality stories and myths, and the leveling influences of adults (most kids do not spend much time around adults who will not tolerate their crap).
50 years ago, Bob and Tim get into a fight. Teacher, Priest, Minister or other local adult takes boys out back and has them fight it out (in a supervised setting, with boxing gloves if available). When the fight is over, the adult makes Bob and Tim shake hands and make up. Lessons learned: Fighting does not solve problems, but it does deal with the emotions of the situation. There are rules to fighting and they do not include killing each other. When the fight is over, you still have to learn to get along and resolve your problems. Adults are in control (enforcing the rules), especially when the kids are not.
If Bob is bigger than Tim, the adult would smack him and say what the hell are you doing picking on someone half your size, and the rest of the kids would look down on him. And God help Bob if it is Jill instead of Tim.
Today, Bob hits Tim, Tim Hits Bob back, both are suspended. Tim is lectured about how he has to tell a teacher if Bob hits him. After suspension, Bob hits Tim, tim tries to tell teacher but teacher is busy, Bob continues to hit Tim until Tim hits Bob Back. Both are suspended again. (Story related by a school employee I know). What have they learned? Tim has learned he cannot resolve his problems for himself. If he has a problem, he has to go to an authority figure to resolve it for him, and they will just ignore him. The system sucks and is stacked against him. What has Bob learned? That he can get away with just about anything with minimal punishment. And the other kids probably look up to Bob for being a badda$$
You want to address violence, sublimation (correct expression) is the key. Taking away weapons doesn't do a damn thing. Suppression makes it worse, just like anything else we try to suppress. We need to transmit a better cultural example to our kids (of any age), and provide a way for kids to express their violent tendencies in a controlled and supervised way (such as hunting, boxing and other martial arts, etc). And we need to remove the cultural incentives to violence (girls, are you listening?).