You guys are like a cult.You seem to know a lot! Even the FBI cannot determine his motivation, but it doesn't appear to be political.
Judging from how he apparently searched for locations where many other politicians were speaking / appearing, including Biden, best guess is he's just your typical right-wing gun nut with a screw loose*, who woke up one day and decided he'd die famous.
*the people the NRA insist that we keep arming with AR15s. Nutters can't be deprived of their constitutional rights!
Murika!