At the end of the movie, Connie accuses Michael of having her husband, Carlo, murdered. He straightfacedly denies it, even though it's entirely true. He repeats the same lie to his wife Kay, as well. Michael is capable of lying to his loved ones for the sake of his criminal business interests—which is totally cold, but the inevitable result of his journey.

Kay watches as Michael's capos (underling bosses) gather in his office. One of them closes the door, and then the movie ends. This signifies Michael's transformation into ruthless crime lord, sealing Kay outside of his inner world.

He's not the golden, studious war hero with whom she was originally in love. He's gone into a different realm, and the closing door symbolizes that he's fully entered into that new world of wickedness. It's a transformation that required a lot of work, but it's placed him, in the end, in a strange and potentially isolated position.