The best theory I've heard with regard to motive is that Oswald, who was a member of the Communist Party USA and who most likely  watched the Walters interview with Castro which contained Castro's remarks that if the US didn't stop trying to assassinate him, he would send operatives to return the favor, decided to take matters into his own hand and return the favor.

One nice thing about this particular theory with regards to motive is that it plays nice with most other theories about the assassination. Whether someone supports the lone gunman theory or multiple sniper theory, the motive could very well be the same. Even if one likes the shadow government military/industrial complex conspiracy theory, they may have found Oswald, intent on furthering the cause of world-wide communism, a convenient pawn.