Deseret News is dropping details as it gets them. Little so far - seems to have a fairly conventional background. College dropout who did one semester, the slogans seem to indicate he was Extremely Online, family member notes he had become more angry and political recently, which also indicates Extremely Online and might indicate few real life friends.
Based in Washington County down on the Arizona border, which isn't exactly the middle of nowhere - St George is a fairly substantial city, biggest in the state outside metro Salt Lake.
I'm thinking back to the Trump assassination attempt in Pennsylvania last year, where the shooter turned out not to be an organised extremist but just some random Reddit kid with poor mental health who had radicalised himself online in a very haphazard way.