I wonder if Islamism isn't just an outlet for disturbed weirdos who don't identify with society, at least in the US. Most of these people were born here, are citizens and everything. They just don't fit into society for whatever reason, and end up identifying with ISIS or Al Qaeda. The lone wolf nutjob white guys have their own causes to identify with, but it seems like similar expressions. You have to really go out of your way to get radicalized in the US, especially since most of their families seem to be pretty ordinary.
Stuff like the Paris shootings are obviously not the same kind of thing.