Kennedy was having an affair with a communist agent. The news was about to break.
If LBJ and the Democrats have Kennedy killed, then LBJ moves up a position to be President and the Democrat Party is saved.
If the Mafia has JFK killed, what do they get?
If the Cubans have JFK killed, what do they get?
If the Soviet Union has JFK killed, what do they get?
The last three face complete annihilation if they get caught doing something like this. And face it, what did the Mafia or Cuba or the Soviet Union have to fear from JFK?
The Democrat Party has made a lot of money over the years from their involvement with government. They had a tremendous interest in seeing Kennedy go away.
It’s like Benghazi.
There wasn't a president that fought communism for another 17 years.
Revenge for the rumored double-cross by the Kennedy family and the end of Bobby Kennedy as AG. Carlos Marcello certainly had the motivation and the means to do this. I don't know if the mob killed Kennedy but it is plausible IMO.