The bible, in the story of Cain and Abel, locates the start of violence in the conflict between static agriculture and people who move around for a living. I feel there's more than a germ of truth in that. Once you you settle and start piling up "things"( physical or mental)., you get pretty possessive about defending them. And the people who move around and are used to living off the land and picking up "things" where they find them inevitably cause conflict when they arrive near the settlers. Basically, behaviour evolved for hunter/gathering may not fit in too well with farms and factories.
   Right, that's my theory on the problem. now, the solution: I'll come with that in a bit.....