So Oswald is in Gomer Pyle-like bliss up to the assassination about whatever is going on. Then it immediately dawns on him that he is going to take the fall for it. He suddenly becomes a genius. So he does the logical thing and kills a police officer. That is quite a fantasy tale. The most logical reason for him to have killed Tippit is because he has just assassinated the President and has nothing to lose at that point. He can't risk the possibility that he has already been identified as a suspect and will be arrested if he IDs himself.
What does 'Gomer Pyle-like bliss' mean?
"If he IDs himself"? Didn't he have a fake ID to show- that said A J or [Alek] Hidell who didn't even actually exist?