Humans are horrible. Throughout the history there's trace of wars. The first war recorded in history took place in Mesopotamia in 2700 BCE so saying it's just one group of people doing it is stupid. Nations have been conquering other nations for ages just look at Genghis khan or Tamerlane in Asia or the Native in America.
The Native Americans were in a near constant state of warfare with one another, many tribes became extinct and disappeared even before Columbus, as a result of tribal conflict.