Dallas was the third go at JFK in Nov. 1963. (Both the now well known, earlier in November 1963, Chicago and Tampa assassination plots were broken up and it came down to Dallas). I agree with the LBJ angle. He was flat out ruthless in his quest for power and his entire life shows it.
Of course without LBJ on the ticket the Democrats might well have lost Texas in 1960...who knows if a Kennedy-Symington ticket would have won.
The LBJ angel has many supporters. One author accused LBJ of also being behind a series of other homicides.