"Us" vs "Them" is a deeply ingrained universal pattern of thought.

You're born into a family: "us." A little later you identify most strongly with your tribe - religion - nation - etc. Outside there's everybody else, some of them scary and threatening, all of them more or less unpredictable.

Realistically, what other mindset was possible?

(Of course the contempt of some Mudcat Brits for Americans - see the "gun control" thread - remains inexplicable by reason.)