Anyone who wants to be a muslim is a muslim. It's more of an identity than a set of instructions or a certain worldview. Wrong or right I don't know but this is the case with those most vulnerable to be recruited by extremists (i.e., muslims in western Europe (and Paris is the best example of that) who have been marginalized and stripped of human value, worth and esteem. They may have never gone to a mosque, they may have drunk alcohol, had sex, etc, but are ridiculously identity-less and confused, and religion suddenly seems to fill the gap). Interestingly, this is also the way west sees muslims. You are a muslim if your parents are. This is not your choice anymore (what we imagine religion to be), this is your race, your nationality, your identity, and you have to carry it with you everywhere you go. You are a muslim, and thus either a threat in the most extreme case, or are at the very least supposed and expected to feel ashamed and embarrassed and sorry because of what other people have done. Not only that, like Zak said, you are bound to be punished for what you have not done. Shitty world, isn't it.
Daesh will disappear (although there are striking differences between their approach and that of say al-ghaeda) but something else will replace them, unless ... unless muslims start getting their shit together? I wish that was the solution, because that would be possible. But Daesh and its likes will never disappear unless the filthy powers who are now mourning over the corpse of humanity (which did not matter when Lebanese people were killed by the same assholes and for the same reason) stay the fuck out of that region. Which will not happen, because there is never an end to their greed.