I don't remember if I wrote this earlier on in the thread, but I'm a convert to Islam so I wanted to chime in with an opinion.
People who join the "State" know exactly what they are getting themselves into, 15 or 51. Now, they may have once been lured by the prospect of no taxes (one of the things promised), gold (another thing they promised), cars and other luxuries (yes, them too), but the reality is this... Everything about the Islamic State as a concept is a self-fulfilled prophecy. How?
Well, in Islam we have things called hadiths which are quotes or things said or done by Muhammad. These quotes were said to have been transmitted by word from people close to the prophet, then eventually compiled by scholars. 99% of Muslims either completely agree with the authenticity of the hadiths we have or at least accept most of them to be what he said, the 1% like me reject all or at least most.
I digress. Of the many things apparently prophesised by Muhammad that would have to take place before the messiah came and triggered the end of times was uprising in the Levant, Syria to be exact. Other religions have prophecies of things that would have to happen before a messianic age could take place, the Third Temple of Solomon being one example.
So when we look at Shemima Begum and say she didn't know what she was getting into, it isn't completely true.
Daesh wanted the prophecy of Muhammad in hadith literature to come true, to trigger the end times, to speed up the apocalypse, the return of Jesus and the Mahdi (messiah) etc. They wanted to fulfill it. Okay, so Shemima may not have envisaged prisoners being locked in cages and set alight, but she would have known she was joining a state created on violence and that many laws would be broken to achieve such a state.