I think mental problems are 95% likely. 20 year old guy in Oregon? Could be a nutjob there who wants to wreak havoc on filthy immos (Oregon is like the whitest state in the US... TIME magazine once
said "In the eyes of a skinhead, Portland, Oregon, looks like the city of the future").
But my bet isn't so much the ideological but rather the personal. Likely some messed up lonely dude who couldn't get laid.