There is an image in Jeff Wayne's War of the Worlds album book (linked below) that haunted me as a child. The look of hopeless panic and the lines that accompanied it:

Never before in the history of the world had such a mass of human beings moved and suffered together. This was no disciplined march – it was a stampede – without order and without a goal, six million people unarmed and unprovisioned, driving headlong. It was the beginning of the rout of civilization, of the massacre of mankind.

And the scenes last night of a mass of people stampeding through the streets evoked the horror of that image once again. It was well done when it was just random people, and then better when personalised via Arya. Who throughout was always Arya Stark, a noble, a trained assassin, empowered throughout her life, even in the face of the Night King, until now, now she was being trampled underfoot by the mindless mob...if only they had followed through on it, it would have been an inglorious yet shockingly brilliant end. I assume GRRM still has some final tasks for her.

As for why Dany did it, I think peoples outrage that one of their fave characters has turned heel, and their ignoring/disregarding all the portents of it's coming is hilarious. The main reason seems to be that it all happened to fast and surely not enough had occurred to make her turn, even if she does suffer from the family madness. But lets consider what has happened to her in the last 6-12months alone (forgetting her traumatic childhood, teenage years, early adulthood) - she has lost 2 of her 3 children (dragons); lost her 2 closest confidantes and friends; lost the man she loves who she sacrificed half her army, a dragon and her closest advisor for (and whos family hate her); lost her 3 allies (and their armies) due to poor advice from her supposedly loyal Hand, who has given astonishingly poor advice and/or betrayed her on numerous occasions, and who keeps siding with his real family; lost her other supposedly true to the people advisor, who she had to kill for plotting against her.

What does that leave her with? Greyworm, who just wants vengence, the Unsullied, who want to finish the war and then probably go home, The Dothraki, who just want a fight at the end of the day. A tentative ally in Dorn, some Ironborn who are sitting on their island, a Kings Landing population who didn't ring any bells until the Lannister soldiers told them to do it, there was no revolution, and the North who dont trust her and want Jon on the throne. She effectively has nothing and no one left, and she sitting on a Dragon and leading a horde, why wouldn't she do it?