I am always suspicious of Chosen Ones, be they Skywalkers, Potters or Targareyans.
That said, I don't think showing her with her hair all witchy and no make up was enough to sell me on her being mad, but you know, women, ey? - Hell hath no Fury and all that - It's true the sense of her being obsessed with getting the throne was set up, but becoming the thing she has fought against WHEN SHE HAS GOT IT was the step too far more than anything, tied in with the level of intended destruction - having her go for the keep and setting off the Wildfire would have been a great moment and acheived much the same thing - a wonderful irony of the daughter finishing what the father began.
Of course, all of this could have been avoided by John Snow not being an enormous prat and waving his birth certificate in everybody's face whilst going "but I don't care who gets the house in the will"