Where did non-native people come from? Somewhere else, where violence and war had existed for centuries. The history of America is about 400 years, the history of Europe is far older and filled with far more violence and war. You can't ignore the fact that the thinking and actions at the time were directly tied to the history of the World as they knew it, violence and slavery included. Violence and war didn't start when the Europeans came to this continent- it had existed for hundreds of years between the native people. The last part of your post should be "People are violent". Some have lost the will to be violent, but I would bet that if the situation calls for it, they could be. Anyone can claim that they're against violence, but under threat, that goes out the window because nobody wants to be a victim.
The stupid thing about wars- it's the governments and people in charge causing them, not the people.