The Pakistani Taliban (TTP) are reactionary and their tactics filthy, but they have in the past been well-rooted enough to recruit thousands for a losing war with the state.  They have enjoyed significant levels of support for their attacks, rising to 20% among Pashtuns. This is in part due to their ability to exert effective political authority over territories - 'law and order' and so on, and in part due to Pashtun ethno-nationalism. But the biggest source of support is Pakistani military attacks and US drone strikes.

The Pakistani military and political elites had been trying in recent years to come to a new alliance with the TTP, claiming that they were really "our brothers" who had been "misled", and so on. This was an organisation which, despite a series of defeats inflicted by the Pakistani military, still had some potential clout.  And it was being lobbied by other Islamist groups such as the ISI-linked Haqqani network to make peace with the Pakistani state - and presumably focus on working with the ISI to conquer Afghanistan again.

In recent years, the TTP lost a great deal of the support it did have due to the brutality of its methods. Suicide attacks on army facilities were one thing. Bombings at mosques, funerals and other public locations were quite another. High profile beheadings also didn't help.

This kind of atrocity, in this context, lacks all strategic logic.  It's the sign of an organisation that has incinerated its popular base and fully embraced the 'takfiri' logic of some other Islamist groups - the attacks on Barelvi Muslims being a case in point.  Its recent expeditions to Syria also suggest that it has shifted from its nationalist focus toward an emphasis on 'global jihad' - and this shift seems to be concurrent with a series of hard military defeats at the hands of the Pakistani army. 

The latest atrocity can only isolate it further and allow it to be broken up relatively quickly.