From one of my commenters, excusing the behavior of the militants in Iraq: They have always been tribal societies held together in various forms by strong, dictatorial, tyrants, warlords and kings.
These leaders served 2 main purposes...to give the people a sense of identity and to keep the enemies at bay.
That is their history, their culture, and all they have ever known.
Here are those poor victims of the culture at work, behaving like the only culture they have ever known:
Ethnic Shiite and Sunni Turkomen raise the grim specter of large-scale atrocities by Sunni militants from the al-Qaida-inspired Islamic State of Iraq and the Levant, or ISIL, who have killed hundreds of Shiites in areas they captured last week.