Only read poorly presented nd constructed conjecture so no opinion either way. The history of political assassination in the US leads me to favor the up front "one man with a rifle." I would have to see concrete evidence of a conspiracy. Neither the "mob" nor the CIA function well under the harsh light of publicity. They shun such as much as possible. I've not read of a "wrong" or a motivators which would make such an action attractive to otherwise secretive organizations. Doesn't add up. I need proof, not speculation by people intending to make money or gain notoriety from their opinions. Getting elected to the presidency means putting on a big target every morning before going out to meet your adoring public, some of whom are not so adoring.
People kill people for, what to most of us are, very flimsy reasons. It's what people do. It didn't happen on my watch nor, in my jurisdiction so I do not give it much thought.
That said, conspiracy theories can be very entertaining at gatherings of like minded people. And the press loves a good theory, especially on a slow news day. Gotta sell papers, gotta pay the help!