??? Pastoralism is a branch of agriculture. Two words: private property In a world of finite resources, it is problematic almost by definition We're not talking about the Australian outback. We're talking about a place where agriculture was possible. "Motive" ? ... well, let me put it this way: Do you know why modern Europeans are not lactose intolerant? Even though the more primitive inhabitants of Europe were lactose intolerant? Because modern Europeans didn't actually come from Europe. They migrated more recently, from the Middle East. People from the middle east, who had domesticated cows and goats had gotten used to milk. Slowly, they started advancing their settlements in Europe and came into contact with the more primitive cultures. But what happened to the older inhabitants? We don't actually know for sure... but the results of that encounter are clear. The older, less advanced culture was wiped out and replaced. That's just how the world works. Finding rational "motivations" in history, is a waste of time. Well, there you go. How many instances do you know where an advanced culture came into contact with primitive one and both got along? Certainly not in North America. Ask the lactose intolerant Europeans, who probably attempted to steal the crops and domesticated cattle of the newcomers, and ended up paying with their own extinction.