There was a strong theory that Daenerys Targaryen would go full-on Mad Queen rage-monster at some point in the series. But tonight it finally happened in truly horrendous fashion. The worst fears of King's Landing, all her allies, and basically everyone except Drogon and Grey Worm were all confirmed: Dany heard the bells signaling that her siege on King's Landing was successful and that the city was surrendering. And yet, her rage and fury consumed her, and she just decided to set the whole city on fire. So...why? What triggered her massacre? In the scenes leading up to her actions, she didn't seem so much insane as isolated and enraged. What pushed her over the edge?
Honestly, there weren't any words during that sequence, so you're forgiven for being a little confused. It initially looked like Dany heard the bells and decided she just didn't care about anyone but herself-which would be channeling madness/egotism pretty profoundly. But according to the creators D.B. Weiss and David Benioff in the "Behind the Episode," once Dany saw the Red Keep, which apparently the Targaryens built during the time of their rule but which had been taken away from them, the unfairness of that huge familial loss washed over her. Presumably, the decades-long grudge for her whole family and herself took over. She took back the city by fire as if she were trying to avenge her Targaryen ancestors-and channeling their craziness, in the process.
If you'll remember, Dany had a vision in season 2 of herself walking towards the Iron Throne, but it was all burnt and ruined. That vision came true tonight: Dany has made the decision that she's going to be Queen of the Ash, and she's ok with it.