The point is that you can't have the degree of pandemonium and conflict between Ruby and his captors in that garage and then the docility right on the other side of the door. It's like putting the North Pole right next to the South Pole. And it just shows that it was all an act. Just think, Ruby was so pacified in the inside scene that reporters asked, "Who's that guy?" They didn't even recognize him.
Good old Inspector Sawyer had to tell them, "That is the man who shot the man." That is the man who shot the man. What does it tell you that he had to tell them that?