Lets pretend Michael was honest with Kay and told her everything that was going on about the family buisness. Would Kay have understood Michael and support him, or would Kay would have been disgusted and left him sooner ?

Kay was part of the same hypocrisy as Michael. She had ample reason to believe that Michael was a murderous crime lord, but chose not to.

All evidence to the contrary, Michael continued to insist that he was working towards legitimacy. He probably even believed it, based on his skewed definition of that term. And Kay kept eating it up with a fork and spoon. If she didn't recognize the impact their lifestyle was having on Anthony, she probably would have continued to delude herself for the rest of her life.

Your question is difficult to answer because Michael never would have done that. But, if he had, I think their relationship would have played out in much the same way.

I agree with Last Woltz. Kay wasn't stupid, she was a champion self-deceiver. I don't mean that to be harsh on her--it's just that she was Michael's willing partner in his own self-deception. Just as he claimeded to be "legitimate" or "trying to be legitimate," she went along, wishing and hoping in his fiction.

And, Pete, recall the scene in GF when Michael returns to New Hampshire after Sicily to woo Kay. He told her he was with his father, then made an elaborte rationale about how his father was "no different than other powerful men with responsibilities..." Kay went right along with it.

I think that the Tahoe shooting pushed her over the edge because (apparently for the first time) Michael's criminal business threatened her and her kids' safety. Note the look she gives him after the shooting when she and Connie are cradling the kids. That was it, period.

I agree with all that's been said about Kay. I believe that she sincerely began to question her life, and the life that she was allowing her children to live in by the time Anthony's communion rolls around in GFII.

There is a deleted scene, which I think backs up this theory, where we see Kay chasing Anthony (during the communion) and Anthony comes upon a house on the compound where the Corleone soldiers and bodyguards are all sitting around. Anthony stops in his tracks and stares while Kay catches up to him. She herself has a look on her face of both disgust and sadness, as though she's realized that she can no longer protect the boy from seeing these kinds of things. As though she realizes that she's only been fooling herself all these years.

The next scene between her and Michael, is the dancing scene where she questions him on what he once told her about taking the family to legitimacy. I believe that at that point she still loves him and wants to believe him, but realizes that it was never going to happen. And then as Turnbull said, later on that evening, the shooting up of the house is what pushes her over the edge.

However, in fairness to Michael ..... It's the buisness she had chosen.

I have said before that as much as she protested, Kay enjoyed the privileged lifestyle Michael gave her. She very well knew what his business was, and I agree with the others that she was totally in denial and deceiving herslf.

Only when her life and her children's lives were threatened did she beging to pull away, then with Michael skipping Christmas to go to Cuba, the abortion and Anthony's decline she decided she had enough.

But as we see in GF III she always loved Michael despite his faults, and Michael, while claiming he loved her, probably merely wanted to possess and control her more than anything else.
If Michael loved anyone it was his daughter Mary. His words to VIncent were prophetic "they come after the ones you love."

I think Kay loved mike, thats why she stays with him, even though she knows what he does. I also think Mike loved Kay until he gets drunk on power and meets Appolonia. Then everything changes.

Like Carmela (GF and Sopranos), Kay could figure things out. But unlike Carmela, she eventually could not bear the emotional conflict she was feeling and the threat to her son.

I don't think that Kay fully grasped what Michael had become. Realize that when they are romantically involved, he has yet to commit murder, he was an outsider to the Family and has no desire to have anything to do with them. He makes that quite clear at the wedding. After he leaves her at the hotel, it is years before she sees him again, and she still remembers the young, idealistic college boy.

And unlike Carmella Soprano, Kay was raised in a totally different world. She probably wouldn't figure things out as quickly or easily, and it might have taken her a long time to truly catch on.

Well, here again, are we discussing the character or just the film? If we are discussing the character, we get to interpret the novel. If so, in the novel, Tom discusses Michael at length with Kay about Carlo and Tessio, etc. So, she was aware. Again, unlike bot Carmelas, she eventually could not reconcile her husband's actions with her morality.

Carmella left Tony (temporarily) because of his philandering, not because he was a gangster. She knew exactly what he was, and derived her financial security from his living.
Michael had many sins, but, apparently, infidelity wasn't one of them. Kay left him because she finally realized he had no intention of ever leaving the gangster life.