Shia worshippers (killed) by suspected Sunni jihadist groups
And there you have the clues for understanding the attack. Remember that most acts of terror are a brutal form of advertising. It is interesting that the media describe the killers as Sunni, not Alquistos, Wahhobbit or whatever their latest boy's club name. Demonize the Sunni and rally support for the Shia. There will be a competing ad soon.
There is a lot more civilization in the Middle East now. There are a lot more liberals. Enough so to trigger revolutions. But are there enough to stop the ancient cycle of Sunni vs Shia.