Then why didn't he run again if he did this for gain?
Great question.
The answer is that JFK was about to dump LBJ from the ticket and was investigating him for a bunch of felonies involving Bobby Baker.
https://en.wikipedia.org/wiki/Bobby_Baker
If you notice all of the key players did not just have something to gain by the assassination of JFK.
They all had a lot to lose if JFK was around for much longer.