A CNN commentary by Peter Bergen, entitled 'Zeros trying to be heroes: what motivates terrorists'. He opens with:
When I was researching a book about Americans becoming violent jihadists, again and again I was struck by how often they were men who were going nowhere fast in life and who turned to violent jihadist ideology as a way of giving their lives greater meaning. They were often zeros trying to be heroes in their own story.The terrorist incidents of the past week in the United States show that this can also be the case for alleged right-wing terrorists such as Cesar Sayoc, who is accused of mailing crude bombs to prominent Democrats and others, and Robert Bowers, who is accused of killing 11 at a synagogue in Pittsburgh.
Both Sayoc and Bowers display some of the same characteristics as American jihadist terrorists: losers who attached themselves to extremist right-wing ideologies that gave meaning to their otherwise dead-end lives.
Link:https://edition.cnn.com/2018/10/30/o...gen/index.html

Just perhaps the latest attacks will be the last, alas I tend to agree with his last passage:
The truth is that zeros wanting to be heroes, motivated by a number of toxic ideologies and armed with semi-automatic weapons, will likely continue to massacre Americans at frequent intervals for the foreseeable future.
I do wonder whether parts of the USG are "clutching at straws" when I read of an academic project is getting (tiny) support ($731k):
to use the Western Jihadism Project’s data collection to create an algorithm that can predict when an individual will become radicalized.
Link:http://www.brandeis.edu/now/2018/nov...n-profile.html

Becoming 'radicalized' does not mean you become violent and from my reading IMHO the tipping point is so multi-faceted prediction is impossible.

A short article from New Zealand, actually originally in the WaPo, 'The psychology of how someone becomes radicalised' and focused on the extreme right in the USA:
For radicalisation to occur, there are three necessary ingredients, according to Kruglanski's research. The first is the universal need to live a worthwhile life - to have significance. People usually satisfy this need through socially accepted means, "like working hard, having families, other kinds of achievements," Kruglanski said. Radicals instead tend to place significance on their gender, religion or race. The second is "the narrative," which gives someone permission to use violence. Kruglanski said the narrative is usually that there is an enemy attacking your group, and the radical must fight to gain or maintain respect, honor or glory. The third necessary component is the community, or the network of people who validate the narrative and the violence.
Link:https://www.nzherald.co.nz/world/new...ectid=12153329