MUSLIMS KILLING MUSLIMS

Because ISIS is killing other Muslims, so the argument goes, obviously, its violence cannot be based on Islam, which bans Muslims from killing fellow Muslims in its name. But the Islamic State does not view its victims as Muslims.  Indeed, mainstream Sunni Islam—the world’s dominant strand of Islam which ISIS adheres to—views all non-Sunnis as false Muslims; at best, they are heretics who need to submit to the “true Islam.”
This is largely how Sunnis view Shias, and vice versa—hence their perennial war.