I have never known of any man who was more widely hated and more intensely hated than Donald Trump.  Add to that the fact that he was running for President (and thus perceived as a threat) and I'm actually suprised that there wasn't an assassination attempt sooner.

This is easily explained as a rogue angry Democrat wanting him dead.

That doesn't mean it isn't something else, but any other theory is going to have to provide convincing supporting evidence that we don't currently have.