That look was pivotal. When she and Michael were dancing earlier, you could hear the frustration and near-cynicism in her voice when she reminded him that he'd said the Corleone family would be completely legit in five years--"and that was seven years ago." "I know, I'm trying," he replies. But the shooting was the last straw--and it occurred within hours of the dance. Small wonder that Kay didn't even look up from her sewing machine after Michael returned hone from Cuba--one of the saddest scenes in the film.