From the look of things, Boko Haram carried out that attack to achieve the following goals:

●Kill GMB and cause bloody crisis across the country.
●Use such crisis to divert great attention of both the masses and the military from BH to other flash points; and then
●Use the opportunity offered by that National Confusion to actualize and fortify their Islamic Caliphate.

Having failed in all of that plot, their present declaration, therefore, is their Last Card!

I believe Bukohari will end up being the sultan of this caliphate and make Shekau his deputy, all na wash.