I disagree. I actually think she had it planned before Ned confronted her. I've always thought this scene was the moment she decided she wasn't putting up with him anymore.
http://www.youtube.com/watch?v=Yh3iLEMG-cA
"How long can hate hold a thing together?"
"17 years is quite a long time."
On first watch, that sounded to me like she's saying "hate has held it together for 17 years," but now I hear it as her saying that this year is the last year it will. Time is up.
She seems willing to be civil with him but when she asks what he wants and he just holds up the wine, she looks extremely annoyed.
"It doesn't make me feel anything."
She is cold now, and finally has what she needed to go through with the plan. She finally truly does not care and hates him.
This was before Ned figured out the incest. He doesn't until next episode. But my reasons are circumstantial I guess.