After each new atrocity by Daesh / ISIS, a solemn murmur can be heard from Muslims and some individuals on the "left": "This has nothing to do with Islam".
This is not true.
Daesh is deeply rooted in Islamic theology, and indeed, Daesh actually believes that the liberal mainstream Muslims who disavow them are apostate, and therefore deserve to be killed.
It is true that many of the young men who go to Syria to join Daesh, or who commit atrocities in the West alone or in small groups are often petty criminals and psychopaths, but Daesh as a movement is a manifestation of a stream of Islamic thought - a branch of Sunni faith known as Salafism, which appeared in the latter half of the 19th century. Salafism aims to return Islam to its 7th century roots, and is analogous to extreme Christian sects such as the Exclusive Brethren and Ian Paisley's Free Presbyterian Church sect.
Al-Qaeda is a salafist movement, but it believes that the Caliphate and the Apocalypse prophesied in the Q'ran is going to come some time in the future.
The USP of Daesh is that it claims the actual Caliphate is happening now, and that Daesh is it.
The argument setting out this case is laid out in detail in the Atlantic here.
The practical importance of this understanding is that it is inadequate for Muslims and their sympathisers simply to say that Daesh violence is "nothing to do with Islam".
Daesh does have a lot to do with Islam and this means that mainstream Muslims are obliged actively to separate themselves theologically from Daesh by declaring a fatwa against those who would join Daesh. This fatwa will not put an end to Daesh immediately, but it may reduce the flow of impressionable young people to Daesh. More importantly, it will frustrate the main objective of Daesh, which is to precipitate a full-scale war between the West and Islam. Instead, the fatwa will set the fault-line between the majority, peaceful Islamic community and Daesh.