Muslims killing Muslims

I'll never understand it. In the news headlines today, yet again, unfortunately: Attacks targeting Shiites kill 72 in Iraq

Sunni Muslims killing Shi'a Muslims. No, sorry, I just don't get it. People of the same religion, with some minor twists, killing each other. And brutally killing each other, too, time and again. I'll never understand that one. Making it worse, they're of the same country, too. It's doubly insane, at least. Just to be clear and rather complete, too, I will never understand Catholics and Protestants killing each other, either, so don't start. Links: http://news.yahoo.com/attacks-targeting-shiites-kill-72-iraq-143525409.html; http://www.truth-out.org/attacks-shiites-iraq-kill-least-60/1325773389; http://en.wikipedia.org/wiki/Islam