Not so much the incentive, but the mechanics during and after the fact. Behavior of Jack Ruby. Every shadow group appears to have had incentive. After the event, none are directly held accountable. It’s almost as if the event was controlled by an upper echelon group.
There is one group not always mentioned with very real incentive. Remember this was after the Bay of Pigs and after other promises to Vietnam. There were many patriotic military and black ops types who may have perceived JFK’s lack of virtue far exceeded that of all the Presidents since FDR. He was Catholic and unfaithful to his wife.
Within those numbers there probably have been some very vengeful leaders believing they were defending a better world by assassination. If placed in the right time and place, multiple players could have acted, with different incentives and perspectives, and remained quiet and devoted to their cause after the fact. Up front sleeper cells, if you will,...commit the act then obediently remain retired.
In any case, the entire Oswald affair had so many real world mistakes that it seems the most plausible.