I have just watched the film for the first time. I found the relationship between Diello and the Countess fascinating. It seemed to me that Diello had lived a life of humiliation being treated as a servant and was determined to escape and become that man in a white dinner jacket, high above it all. But the Countess, rich and respected as she was, had lived her whole life as a woman of her era. That meant that, no matter what her status, she would always be subject to the humiliation that came with simply being a woman. Even Diello could never understand what that was like -- to be rich and titled and yet never truly to have any power, except what she could obtain from her influence over men.
I can't be sure whether she ever had any real feeling for Diello, whether she intended to betray him from the start, but I think the turning point was when she had gotten all the false papers, withdrawn the money, using every ounce of feminine charm, as she reported to him, and she said "we have enough" [money] and Diello said "we?" and let her know in no uncertain terms that she would only get money from him at his pleasure.
I doubt that any of the men writing the material on which the film was based or otherwise involved in the production could have understood the Countess' position as a woman as a modern viewer can. The Countess herself would not have understood it in modern terms, but she knew that she had only the power of her sexual allure to count on in a world where men held all the cards.
One does root for Diello because of Mason's sensitive portrayal and I wished that he could have remained on his Rio balcony, especially since his treason had essentially netted the Germans no advantage (love that hangman speech!) but I suppose his bitterness at Anna's betrayal would have continued to overshadow his triumph, so his vengeful laughter really is the perfect ending to a splendidly subtle film.