I always thought it was a look f total resignation, and knowledge that no matter how much Michael "won," that in the end he "lost." It was one of the few moments in the trilogy where Michael is not in a state of denial about his "legitimacy."
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
"I stood in the courtroom like a fool."
"I am Constanza: Lord of the idiots."
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