Originally Posted by juleska
I think it's that he so loathes the nature of the politics that he deliberately acts against them, as if to force the players to bend to the more honorable rules of the battlefield.
I think you hit right at the heart of it with your last sentence. I think that like most classic tragedies, what gets our hero into trouble in the end is hubris.
Taken from this angle, it was Ned's overconfidence in his (ample) abilities and power that led to his downfall. He was so sure of his ability that he gave Cersei warning of what he would do in order to give her fair warning to run and hide her children. I don't think it was stupidity, just overconfidence.