You do not appreciate the depth of the Sunni-Shi'ite schism.
It goes back to the death of Mohammed, and became ugly very quickly.
Osama Bin Laden (who almost certainly sent these maniacs) believes that the Shi'ites are heretics, enemies of true Islam, just as much as the Jews or the Americans. You can see this easily if you look up some of his quotes about Islam.
The religious fanatic is most dangerous to his co-religionists: They are heretics, while the rest of us are only infidels. Heretics are considered worse.