When did Margaret Ford know she was going to kill Mike?

In House of Games, the Lindsay Crouse character (psychiatrist Margaret Ford) seems to be unaware of her being the mark until the red convertible shows up unexpectantly. She then performs a rudimentary investigation by visiting the tavern and overhears the recap of the con by Mike (Joe Mantegna) and this co-conspirators that solidifies her suspicions.

I suspect she knew she was the mark much earlier than that, since it would be much too difficult to devise a counter-con (the elements in parens below) that quickly. It was already night when she heard the conversation, had to develop a plan before 10 PM, and execute it at the airport:

A bank story and a bag with ($250,000 cash),
A convincing story of being followed ("I'm frightened all the time"),
The gun had to have bullets (Did her suicidal "patient" really give her a loaded gun?),
The accusation and (her confidence in Mike's bravado - false or otherwise),
Mike turns against her because of her ("crap out of turn"),
"You're all bluff" - ("It's not my pistol, I was never here")

So, how early in this counter-con did Margaret know she would kill Mike?

Best Answer

She sees Billy (her patient who is in on the con) leaving in the red convertible. She then goes to the bar where she overhears the plan and then sees that the "dead cop" is really one of Mike's cronies who was in on the con the entire time. For some reason, while announcing that the "crazy broad" (Crouse) stole his lighter, Mike tells his cronies about his travel plans.

Crouse then follows Mike to the airport and attempts to "con" him with a tale about her having $250k on her. Mike hustles her into the baggage check area so that she won't attract attention , where she continues with her attempts to con him. She slips (shows her "tell") when she mentions that she has his (Mike's) lighter. Although Mike doesn't know how she knows it is his, he does know that she should not have known that fact.

Sensing that he is being bluffed, Mike attempts to intimidate Crouse. When she pulls a gun to threaten him, he continues to bluff her,tries to escape and he is then genuinely surprised when she shoots him.He disparages her as being a "violent" criminal when he is above such things.

There was never any intent upon Crouse's part to "con" Mike. She planned to shoot him all along and she was hoping that he would provide her with a justification (in her mind) for her actions. Mike recognizing her "tell" and attempting to walk away simply hasten the actions which she had already planned.