Hi Mustache Pete how are you? Michael betrayed Kay from the time they were dating saying "That's my family, that's not me." at Connie's wedding. He stayed with that same delusional belief that the Corleone family will one day be legitimate. He knew how the game would play out. Yet, he lied to her to convince her to marry him and be the mother of his children. She was quite naive too and trusted him. He betrayed her. He was never straight with her.