I knew a Vietnam War veteran. He was a sniper for the Army. It drove him partially (if not fully) insane.

Even though the war was over in 1979 when I met him in Germany, he was searching for something, someone to fight against. He was wired like that. He became a Jaeger, a German royal huntsman, but that wasn't satisfying. He talked about becoming a mercenary and going to South Africa to support the white colonialists who wanted to continue apartheid.

He would try to "explain" and justify the Third Reich. Hitler was a visionary.

And being an alcoholic didn't help. He eventually returned to his family in the States, who didn't understand him and how he had changed. He couldn't get employment. Every evening, he would be in a small bar run by a high school friend. From "his" seat, he would watch FOX and boisterously endorse all rightwing positions. Democrats were a danger to the country, etc.

His family worried as he had multiple firearms but they were afraid to take them away from him. He once aimed a loaded rifle at his young nephew when he tried extort more money from the youth regarding the sale of his vehicle.

Though not said aloud, there was relief when he died from excess smoking and drinking.

I have no doubt that had he lived, he would have been in D.C. on January 6, 2021.

and state that this extreme behavior does not represent typical 'military mentality.'

And - while lots of vets were, and are, plenty messed up by their experiences - the 'extremism' that the OP points to - has its roots in other pathologies. Most vets with 'problems' - are far more danger to themselves than anyone else.