At last an interview with two UK citizens who went to fight the Assad regime in Syria, not joining ISIS or al-Nusra and who have retiurned:http://www.buzzfeed.com/husseinkesva...reign-fighters

Here is one quote:
...vulnerable young people, especially those who aren’t really practising [Islam], will be attracted to them. The hype about fighting Assad is over, and larger groups like ISIS will have an appeal to these guys – they are in a vulnerable state, and they’re like a sponge, they’ll believe anything.
Another, with my emphasis:
You have all these groups talking about why young people are becoming radicalised and joining groups like ISIS, and there’s no one who can really tell them why what they’re doing is wrong. The groups who are currently talking about deradicalisation have no credibility – you need someone with on the ground experience of the conflict, and people who also believe in the idea of proper jihad … so that you can tell young people what they’re doing isn’t Islamically authentic.