This sort of trouble doesn’t require a religious difference. Ethnic differences, when all share the same religion at least pro forma, can be the spark.

“Economist” magazine reports

ETHNIC warfare in Pakistan’s most populous city has reached such a level that Karachi’s ambulance service now has to send out a driver matching the racial make-up of the destination district to pick up the victims of gang attacks. Otherwise, the district’s gunmen will not let the ambulance through. Now ambulances themselves are coming under fire, as gangsters try to stop them saving the lives of their enemies. Karachi’s ethnic wars have claimed some 1,000 lives this year, with more than 100 in the past week alone. By contrast the Taliban and other religious extremists kill tiny numbers in Karachi.

Emphasis mine.

Any level at all of this stuff is too high, but some perspective is helpful. Short of remaking human nature, at a certain background level of “radiation”, all you can do is make sure everybody understands that it’s illegal, then resolutely jail the punks forever, or execute them if anybody dies.