Johnson was a power hungry man who would do anything to be president. He realized that he was never going to be closer than VP as long as Kennedy was alive.
Robert A. Caro doesn't say this but while reading LBJ's bio’s this thought struck me like a bolt of lightning.
Most politicians are 'power hungry' but that does not make them conspire to kill the president.
And if LBJ was that damn obsessed about being president, -- obsessed enough to orchestrate an assassination, why did he just step down in 1968 instead of running for another term? He probably would have been re-elected if he had run.