From what I've seen, there is no way to substantiate any claim that JFK, Jr. was a victim of anything other than an accidental plane crash.
That is one where JFK Jr probably did it to himself.
I am a pilot, so I know some about learning to fly.
He did his training in a Cessna 182. It is a forgiving airplane that doesn't fly too fast. I did most of my training in a Cessna 172. It is slower than a 182, but handles about the same.
I later bought a Cessna T210. It is a high performance airplane quite similar to the airplane he crashed.
He did not have many hours, and very few in the high performance airplane. He did not have much instrument time and did not have an instrument rating.
I have an instrument rating, so I know what it is like to fly an airplane in the dark with little visibility.
When you are over water as he was, the stars reflect off the water and so you cannot see the horizon. You cannot fly with visual references, and must use the instruments.
He was flying toward land and had few lights to use as reference. He got disoriented, and the airplane rolled upside down. When flying, you cannot tell which side is up without either visual reference or instrument reference.
The airplane went into the water upside down, which would shear off the top of the airplane. When they showed pictures of the airplane in the water, when they found it, it was still upside down.
When I transitioned from a lower performance airplane to a high performance airplane, it was almost like learning to fly again. That is why a pilot cannot fly a high performance airplane without the necessary instruction and endorsement by an instructor certifying that you were able to fly the airplane.
He had not flown the high performance airplane without his instructor before this flight.
He got a later start than he had planned on, and should have canceled the flight. It was beyond his capability.
After more time in the airplane, and more instrument training, he could have made the flight without a problem.
As a comparison, would you get into a high performance sports car and join the freeway traffic when it was foggy, and you had not driven the car before?
If you did, you may not be too successful either.