It's funny how human relations so frequently follow this pattern:
Group A contains individual 1.
Group B contains individual 2.
Individual 1 has a bad day, is an asshole to individual 2.
Individual 2 then concludes that Group A is full of assholes.
Individual 2 is an asshole toward individual 1 and the rest of Group A as a result.
The rest of Group A now feels that Group B is full of assholes.
Group A starts treating everyone in group B like assholes.
Group B feels justified in considering group A to be assholes, and starts treating everyone in group A like assholes.
Group A feels justified in considering group B to be assholes, and continues being assholes to group B.
Rinse and repeat.
The only time this pattern is broken is when you have people who know each other and trust each other despite the A-B group division, and people are in a better mood and are willing to realize that they're all just people in the end.
Of course, some people aren't willing to look to their better selves, but instead just get stuck on the "Group A is like that" or "Group B is like that" stage.