But you have been telling others that they cannot assess an ideology by the content of its scripture, only by the behaviour of those living under it, which is demonstrable nonsense.
So do you actually believe that it is "demonstrable nonsense" to assess Nazi ideology by the factual policies implemented by factually existing Nazis?
Is it "demonstrable nonsense" to assess the French Revolution by the factual policies implemented by factually existing revolutionary leaders, rather than the pure, abstract ideas of the Enlightenment?
Is it "demonstrable nonsense" to assess the factual reality of factual Bolshevist policies, rather than focusing entirely on the abstract philosophical theories laid out by Marx, Engels et al?
Of course you don't believe that. I predict that in the face of these examples, you will immediately backpedal and claim that no, really, these factually implemented policies were, in fact, visible from the very start in purely ideological texts, even if you are completely unable to come up with any evidence in support of that claim.
Also, the behaviour of people is influenced by the ideology to a greater or lesser degree so one cannot properly understand their behaviour without understanding the nature of the ideology.
Is it though? You claimed that you wouldn't be able to tell to what degree factual bigotry of Muslims is influenced by the bigotry of Quranic verses, did you not?
So from where do you draw this sudden certainty that you can trace the exact influences of millions of people's behavior solely from reading a sacred text?
Are you sure you are not, in fact, working backwards from the premise that your approach of pure textual exegesis is intrinsically worthwile, and therefore must be the key to understanding every ideology and religion in human history?
When ISIS use female captives for sex, it is simply incoherent to claim that the explicit permission to use female captives for sex in the Quran and sunnah has no influence on that behaviour, when part of ISIS's raison d'être is to impose a literalist and retentionist reading of those scriptures.
All of these practices can be readily traced from factually verifiable material and cultural conditions - Muslims are, after all, not the only people who have historically forced women into wartime sex slavery. In order to understand ISIS and its policies, I see no need to pore over thousand-year old texts
that you yourself have admitted the majority of these people haven't even read properly.