As I always say, the funniest possibility is either the official story where some massive fail guy stumbles through life and is passed around because no one even wants to deal with him until he stumbles into assassinating the president, potentially in the process of failing to assassinate the mayor, and then another huge fail guy decides to assassinate this fail guy for the purpose of "sparing the first lady having to look at and speak to her husbands killer". Either that or the theory where a hung over secret service guy forgets what a safety is for and blows the presidents head off after being startled.

So what if its actually the deep state-mafia-gusano collective behind it all, thats boring as fuck and every story goes like this when it comes to American conspiracies, lets have fun since it literally wont matter either way and we wont find out for sure unless some documents proving it manage to escape the shredder.