It’s often the way with minorities that perceive themselves as being on the wrong side of a border. Croatia was also fertile recruiting ground for the Nazis, thanks to a generation of professionally aggrieved Croats who wanted to get even for being made part of Greater Serbia (aka Yugoslavia). I believe the Magyars in Transylvania were unusually nationalistic when Hungary (briefly) took them back from Romania in 1940. Here in the New World, the French-speaking population of Quebec gets away with a habit of openly festering racism that would not be tolerated if they were not able to play the victim card. (Yes, they’re still up in arms about having been conquered by Britain in 1759. The fact that France didn’t want them back does nothing to sweeten their mood.)

This doesn’t constitute an excuse for anyone’s ill behaviour, by any means; but it does to some degree provide an explanation of the psychological phenomenon. People will do evil for revenge (against real or imagined wrongs) that they would never do for mere fun and profit.