This is an interesting subject in it's self. There is a large body of evidence to show that it is only in the last 4,000 years that we became such a destructive species. People imagine going back that humans were a lot more barbaric and uncivilized but there is strong evidence to show that this is not true. War between tribes was rare and killing people in your own community was even rarer.
Back in time it seems we were a lot more in tune with nature and each other, we were more instinctive animals and didn't have such a sense of self. Our communities were rather like the other monkey species on the planet, males would maybe wrestle or fight for mating or leadership rights but killing was rare.
Eventually though came the dawn of self realization. A greater understanding of our selves and the world also brought a new fear of death. To escape death people tried to gain power and possessions as a means to try and transcend it, the dawn of the ego was upon us. This lust for power, control and possession eventually turned into war. Human war more than anything then shaped the planet to where we are today.
So yes our move away from nature to a one of self realization and knowledge has definitely been a double edged sword.
Sorry to be so off topic....
Okay I have to deal with what you are saying here.
You are saying that we as early humans were all lovey dovey and never fought? I find that really hard to believe. Infact even as far back as 4000 BC were we enslaving people to build great pyramids. (They could not have been built without mass enslavement.) Also had prehistoric man been given a gun he would surely have shot his enemys.