Europe has a long cultural history where behaving that way is cool...
Once I taught a class where we read some stuff about WWII and probably covered some of the fighting in Serbia.
One of my young black students, a sweet guy who even cities like New Orleans can produce, came up to me after class one day and said "I didn't know white folks could be so prejudiced against each other."
Then I thought about the long history of mutual hatreds in Europe and felt sad.
When the Nazis got desperate they recruited Bosnian Muslims into the Waffen SS.