Someone asked me why we say that the Jama'at ISIS are Khawarij having extremism in Takfir. I replied to him in haste as follows. And I only mentioned a short portion of their deviance due to lack of time and did not bother going into detail.  

They have the Aqida of the Khawarij such that they make Takfir on what Allah did not make Takfir on. Such as the Amjad media video release which makes Takfir on Lawazim after showing a Muslim sitting with a Kafir and other than that from deviance was showed by ISIS soldiers to the tribes of Halab. And this action of the ISIS soldiers were showed on Furqan (their official media) 

Other than that, there is accusations of Kufr upon Muslims by Furqan such as them calling Sheikh Abu Hammam al-Suri a dog of the Sahawat for giving a testimony against them. Also Adnani said Sheikh Ayman believes in Democracy and majorities when Sheikh Ayman said that the Ummah can chose their leader and al-Qaaida is with such a leader as long as he meets the Shar'i requirements and rules by the Shariah. Here Adnani did not separate between the Ruler and the Hukm by which he rules. Choosing a ruler is not same as choosing a hukm/shariah to rule by. Also when Sheikh Ayman spoke in the context of temporary covenants or taking Jizya, Adnani understood this as taking the disbelievers as Awliya hence he accused him of taking the Hindhus, Rafidis, etc as Awliya and this is a small example of J.ISIS's understanding of what is not Kufr as a Kufr. Also he accused Sheikh Ayman of praising Mursi while Ayman actually called him to stick to the Shariah and IF HE DOES STICK TO IT, he will be victorious says the Sheikh. But Adnani understood this as praising the disbelievers and their Mazhab of Kufr. 

And other than that from countless deviance. As Sheikhul Islam Ibn Thaymiyya said, the basis of the Khawarij is that they understood what is not a Kufr as a Kufr. 

Similar to the early Khawarij who said that ruling by other than what Allah revealed is a Kufr which is true, but they understood that the sending in of an arbitrator means ruling by other than the Shariah. So they took one action and understood it wrongly, placed it elsewhere and ruled on it. This is exactly what Adnani and his group is doing.

Bilad al-Sham Media 

@BiladAlSham_Dhi