Going back to the matter of the three girls under discussion.
I'm sure this has been said but my view is that they are of an age and intelligence level where they become personally culpable for their own decisions, at least to an extent. They are of course at an impressionable age but nevertheless they are also at an age when they will understand right from wrong. They will surely understand the nature of murder, they will know what terrorism is and it's possible outcomes and consequences. Their decision to leave this country to join a brutal terrorist group was not a spontaneous one. Their journey will have been many months in the planning, throughout this period their mindset remained constant - in other words it was a long considered decision. I will also add that teenage girls generally find it very difficult to keep things to themselves and if their lives have been sheltered then planning a major relocation probably didn't come easily either. Therefore I find it hugely improbable that a number of other people did not know about their plans. Those people clearly failed to report this to the authorities and must in some way have aided and abetted the girls. I also find it very hard to believe that their families knew absolutely nothing of their activities, but of course that is simply speculation.
I've read that when recruits join extreme terrorist groups they are expected to prove themselves immediately, often via some sort of initiation process. I wonder what that might have been, for the girls. Perhaps they were handed a gun and told to execute a hostage, or similar. I also suspect that once you join an extremist organisation you can't simply go home when it all becomes too much - you're carrying a lot of inside information and the group will likely regard you as a spy, and so the chances of you leaving alive are probably fairly limited.
On the matter of religion - I'm of the view that religion breeds more division, mistrust, and discord than good. Ancient and modern history supports that amply. I have also experienced religious division firsthand because I've lived in many parts of the world throughout my life and I have also seen religious judgement and bigotry here in the UK - my positive experiences have unfortunately been few. In light of my experiences (and also the fact that I see no evidence to support the existence of deities) I have chosen not to be part of any faith. However, I believe in the freedom to choose one's beliefs, but I cannot help feeling that the world would be a better place if many religions did not dictate or impinge upon the company we keep, where we might live, who we might marry or divorce, and the rules we're subjected to. Religion has always been about power, and power breeds corruption and contempt - religion also breeds hypocrisy and extremism. Of course there are good people in every religion, but there are far too many who will use their faith to justify their actions - actions which are often unpleasant, repugnant, illegal, or destructive. I think the world would be a better place if religion were a private matter, not something forcibly foisted upon those who may not share the same set of ideals, because one religion is always going to be 'better' than every other.