I like this episode and for some reason the burning / sacking of the city wasn't a surprise to me. Like I have always felt that this freeing people at first was a nice act of her but after that she became obsessed and somewhat delusional that she was this Messiah / Mother character, easily understandable with what happened. Remember her being carried and cheered like Jesus and the ending with her in the middle of an ocean of people. It's her fate to free the world - by any means possible.
With this as a backback she went back on her main quest - freeing Westeros and reclaim the Throne from the tyrants who stole it. This is 50% a pure revenge mission. Only thing is, she is not being welcomed as the saviour she had expected (her brother always said how the people long for the Targaryens to come back ), even after being the reason they won the war against the WW - she is still being viewed as an imperialistic invader coming with known rapers and super weapons. Slowly, one by one her advisers either dies or betrays her, her only real friend gets executed and her love rejects her. After the scene with Jon she decided that Westeros is - will not be like she had imagined and since KL is such a symbol of Westeros, in a mental state of rage and vengeance she burns it down. For me it is, fully understandable considering her personality. She dehumanize KL to a 100%,
For me this whole story about Dany is that even the purest intention can bring the greatest destruction.
The white walkers represented one way of destruction - the cold obvious threat. Ice
Dany represents the suttle, harder to see coming destruction. People who think they are something they are not and who got way to powerful weapons at their disposal. They will snap under the right circumstances and if not guided correctly. Fire.
A song of ice and fire.
Though, I don't really get how Jon is supposed to be the balance in this. He obviously knows nothing and never has. Maybe he is supposed to represent the naive human race that never understands the real threats until it is too late.
