My point is: it's easy to find flaws in the reasoning of extremists, and use that to show how "crazy" and "illogical" and "insane" they are, and thereby distance ourselves (mentally) far from them. What we need to do however is face the much more scary truth that even the people who commit atrocities are for the most part reasonable and (somewhat) intelligent people, with some sort of roots in "normal" society, who for some reason just get one idea in their head very strongly (and act on it), and this can happen to many (I won't say "all", but I would say "most") people.

This sort of cases is not about isolated loners that we cannot possibly understand; freaks of nature who are not like the rest of us "normal" people. They can show up anywhere for reasons and through processes we just don't quite understand. But we have to not delude ourselves and see them as an "out group" - radicalization can happen to many people you would bump into in your street, every day.