The magic bullet is a joke...
Motive? How about almost everyone: Mob/CIA/Military/Oil/Cuban refugees.

Power...not sure the consipracy and coverup are the same.

There was alot of stuff they wanted to keep quiet and that helped bury alot of info. Also LBJ wanted an open and shut case against Oswald to calm the nation down as well.

It is a shame they have not disclosed all they know.