Originally Posted by drhill
You are absolutely correct, but her actions did speed up Cersei's reaction and did not help Ned's neck. Though it could have been inevitable.
Well, it was never Cersei's plan to kill Ned -- she wanted him out of the way, but understood that killing Ned probably meant Tyrion's death, and PITA or not, he's still a Lannister and still her brother. She was perfectly happy for him to take the black. Ned's death was the result of her inability to predict Joffrey's stupidity and attempt to assert his independence.
At the end of the day... Sansa is a young girl who's led a sheltered, protected life. She had no reason to expect that others around her didn't have her best interests at heart, and defying Ned was simple teenage rebelliousness.
If you look back far enough, even before the incident with Bran, whoever killed Jon Arryn is ultimately the person or persons who set all of the events in motion. Had he lived, Ned would never have gone south in the first place.