I think you are onto something. Let's see what the US Government says:
Bullet points (I skimmed the report):
* Identity and a sense of belonging are key ("radicalization" which is poorly defined, results from isolation, frustration, and hopelessness)
* Exposure and radicalization are not necessarily correlated (I didn't get a sense of what the key risk factor is...perhaps natural propensity to violence? Maybe this explains why it's young men and not young women who are almost always the perpetrators. We have a rich cultural tapestry to draw on to make more "common sense" conclusions here.)
* Intervention and deradicalization are possible, but difficult (made more so by something not contained in the report: lying to an FBI agent is a federal crime in and of itself...just ask Martha Stewart).
* The role of mental illness is overstated.
* Prosocial modeling (with a heavy reliance on principles drawn from Cognitive Behavior Therapy) are a plausible solution.
* The role of family and a support structure is key. (This part is almost circular and discounts natural adolescent rebellion; kids reject their parents values as a matter of routine.)
Well, it's an interesting read, even if it has some shortcomings. The FBI has a new class of domestic terrorist, the "NVE" or "nihilistic violent extremist." (Not mentioned in the report, very relevant in this case.) It also has the impossible task of preventing crime before it happens while balancing civil liberties. The report opens witn a quote from Christopher Wray that states something like "We don't deal with ideology, we deal with violent acts." OK, organization that had several secret files on MLK...It also contains an unattributed quote that reads "I was raised by the Internet." Overstated as it applies to everyone born after 1995 and I can only assume that the attribution was left of because they did not believe there was a person out there who calls himself "Childish Gambino." Thought-provoking uestions about what motivates attacks like these abound, anyway.
Tldr; you are right, this isn't a left/right thing...we now return to our regularly scheduled program...