Motivation: MAKE A NAME FOR HIMSELF. TO MATTER, FOR ONCE.
This is the classic lone-wolf, right wing extremist, gun nut that law enforcement has warned us about for years.
He was a Republican
He was into guns and gun videos (note the t-shirt)
His high school classmates describe him as definitely a conservative
He is described as relentlessly bullied.
(He did NOT donate $15 to ActBlue)
Let's accept that the right wing has been correct about something: We have a problem with mental illness. They say it is mental illness, not guns. I'd counter that it is the combination of mental illness with easy access to guns, a lot of young people that feel marginalized, and a desire to lash out against society.
I think this kid would have taken a shot at Biden as well, given half a chance.
The only thing notable about this is he was willing to take a crack at a fellow Republican, but that just means that a desire to go down in history was more important than political party to a mentally ill young man.