Kay was shocked and horrified at the end of GF when she realized that all of Connie's accusations were true--that her husband really was a murderer and a Mafia Don. Michael had told her never to ask about his business. I think she used it as a rationale for accommodating herself to the situation, rather than leaving him or filing for divorce. And, things seemed to settle down after they moved to Nevada and Michael became a "legitimate" businessman and "public benefactor" of the State University.

But, the influx of Michael's gangster associates at Anthony's party upset her and reminded her that her husband was still in the OC life (you see this in part of a deleted scene). She reminded Michael, while they were dancing, of his pledge that the Corleones would be "completely legitimate in five years--that was seven years ago." And then, bang! The bullets started flying. She was already at the end of her patience--the attack sent her over the edge.