I think I can give you some background as someone who grew up around Islam.
You might think this is twisting the narrative but I know that a lot of people do/did think like this. Establishing a caliphate has always been a big political thing in Islam, a faux notion of a muslim 'utopia' going back to the first Caliphate right after Muhammad died. It's seen as a way to unite the scattered muslim community and for a lot of people to right the injustices being done to Muslims around the world (e.g. in Palestine, the Rohingyas, the Uighurs, in India; and Chechnya, Kosova, Bosnia etc before them..). Now obviously we know what Isis is and became, but I remember when they first declared a caliphate I read a popular Islam forum online and there was a lot of I guess sceptical hope that it could become something good. In theory it's what some people do want, obviously the application was horrible.
Now I should add - this politicisation of Islam is somewhat controversial - the majority of Muslims I know see it as a personal religion - but there is the grey area where it becomes political and that's where a large part of the terrorism comes from. Unlike Christianity which went through a long (and often bloody, e.g. the 30 years war) secularisation process, Islam never really had something similar. Even Judaism has reform sects - Islam as it is now is still largely what it was a thousand years ago.
Now the question would be why did Shamima go join Isis? I would bet good money that it's because she thought she was going to build a good Islamic society - not because she'd get to commit atrocities. Of course the fact that atrocities had been committed by the time she had joined and I think she said something like she wasn't bothered about the killings work against her - maybe she is a nutjob who deserves abandoning. But I think it's much more likely for 3 kids collectively to join for a positive cause than one that's murderous. You have to take in to account the grooming aspect and what messages she's likely to have received during this time.
On the guy in Reading - again to give an understanding of the other side, they usually see it as a form of retaliation for attacks on the Muslim community, e.g. against Iraq, Syria etc by Western forces. It again ties back into the whole politicisation thing - whereas British Christians will largely see them as British as a nationality and Christian as a (private) religion, some Muslims will see themselves as Muslims first and foremost. That combined with bad personality traits, poor environments and probably forms of radicalisation lead them eventually to terrorism - had they been born white with the exact same character the worse they'd probably get up to is robberies or hooliganism.
What can you do? Thankfully it's only a select few it affects, but I've long though a form of reform in Islam to strip out the political bits would be useful. It would strip out the long-form educational process of radicalisation that eventually trickles down into terrorism. The best bits about religion are usually personal and for the community - not for building global states. But I don't know if anyone really has the appetite for dealing with that process and the backlash they'd receive for trying to so.
