Of all the kooky claims supporting a conspiracy, the "mysterious deaths" justification is among the most absurd. Imagine the roving bands of deaths squads knocking people off left and right. Sometimes years or even decades after the assassination. Long after almost all of these people have told the authorities everything that they had to tell. And the death squads add untold numbers of new conspirators to carry out these acts. These people would then need to be silenced by someone else. And on and on exponentially until there would be no one left to bury the dead.
This is like the US was/is a banana republic with groups vying for power killing one another. Presidents being assassinated in broad daylight with hundreds of people watching, witnesses killed, staged investigations led by corrupt judges and controlled media. It's the United States of Guatemala. Only this time secretly. I know we've got problems; the Establishment (such as it is) has been awful but this is really over the top.
In conspiracy world it's conspiracies all of the way down; that's how they explain what has happened since that day in Dallas. The claim isn't that 60 years ago a small group of rogue CIA agents with a handful of others, e.g., anti-Castro Cubans, mob types, conspired to kill JFK. And then the Warren Commission and others missed that act. Out of shoddiness or incompetence. Okay, that's plausible. The claim is that major elements of the CIA - from Angleton to others - conspired with other groups to kill JFK and then frame Oswald. Then the WC covered it up. Deliberately.
Then the news media covered it up. For more than half a century. The followup investigations have covered it up. Then people like Seymour Hersh and Tim Weiner have covered it up. It hasn't been incompetence; it's been a series of deliberate acts by multiple generations of Americans inside the government and out. Add to this mix of conspirators the roving death squads (Marina is still alive: I would think the conspirators would want her dead first? She could reveal how they forced her to testify against Oswald among other damaging revelations).
This is silly. You have to have a bizarre mindset to think this is possible. A Hitler or a Stalin couldn't do this and keep it quiet. But the US government with its bureaucracy and leaking and free press can do it? C'mon, this is a fantasy world.