A trait of all humans seems to be the need to classify ourselves as different, better than others. We align ourselves within our family, gender, race, nation, region, religion, sports team…
This need to classify, judge, and oppress in the social hierarchy seems to be universal. Even in the most egalitarian of societies there are subtle ranks.
This us against them mentality has lead to warfare, genocide, and torture. People are willing to get into a knock-down-drag-out fight over which team is better. It only gets worse as the issues get larger. Humans seem to fear and distrust those that are different from what is their local area or group. You look, act different then the people here, therefore you could be dangerous. The “Good Guys” versus the “Bad Guys” you simply redefine who is good and bad depending on your perception and opinions.