Seems like most of the violence (note I say most, not all) in Iraq is between the Sunis an Shia Muslims. Ever wonder why? There are many reasons but I think I know the main reason they hate each other. I may stand be corrected here because I am pretty clutzy with these culture things.

The Shia believe in Nikah Mut'ah which is a temporary marriage. The Sunni consider themselves more orthodox and are opposed.

So for that people should die? It's even worse then the Hatfields and Mcoys, the Democrats and Republicans, the north and the south.

Hezbollah is Shia and Hamas is Sunni so they are fighting with each other about how to kill the Jews.

Here are some more differences. And here.