Your theory is possible, given your evidence. IOW, this assassination attempt wasn't all that politically partisan, maybe he intended to shoot both of them and Trump showed up first in his neck of the woods. Obviously he didn't know or understand the chances of surviving an attempt to shoot either man, which means he was quite ignorant of that reality but that doesn't have anything to do with his motive.
Meaning no offense, but whatever his motive might be, it isn't as important as the fact that the Secret Service fucked up. It isn't right to smear an entire group of people based on the actions of one person, and an imbalanced person at that. So, I don't care what his political views were, there are no doubt quite a few people like him out there, on one side or the other. But we cannot allow somebody to take a shot at Biden or Trump before anybody shoots at him or her. That shit has to stop right now, if you point a gun at either man, you should be 'neutralized' before you get a shot off.