Many, many years ago, as a young man, I was hitch-hiking east from Texas. A car, right outside Dallas, picked me up. It was driven by a Texas Ranger who drove me all the way to Memphis. He was going there to pick up a prisoner to return him to Texas. This Ranger, who's name I long ago forgot, had been involved in the Kennedy investigation. I aked him his thoughts and he believed Oswald to be the lone gunman, but he also believed Oswald was working for someone. When I asked him "who", he replied, "Who's the only person that gained anything by Kennedy's murder?" "Johnson", I said. He smiled, nodded and said he was convinced that Johnson set it up. I've always wondered since if he was right.
Far more important reasons than just beoming president were the motives for Rasputin's evil twin, LBJ.
Here's a plug for Barr McClellan's book on why he believes he has the goods to prove the scum LBJ had it done. I certainly remember those days of Bobby Baker, Billie Sol Estes, et al. The books, title is "Blood, Money and Power" I believe.