We learnt a lot since 9/11 about how radicalization works. It's not happening mainly through personal trauma, but through indoctrination, usually through schools and universities.
Sageman’s “bunch of guys” theory has really won out now that anybody can interact with terrorists on social media. You can’t run a big terrorist organization made up of people seeking revenge because they’re power-seeking and will replace you. Instead you take a bunch of guys out of college, give them a way to seek status that conveniently involves them dying before they can replace you, and then go out and fundraise.