I think the main explanations about Dany don't address one of the main reasons why I think Dany made the decision to burn Kings Landing to the ground.

Most people cite her previous actions in burning other cities and committing terrible deeds to (usually) terrible people, those actions getting more gray and darker each time. While this certainly pushes the threshold of what she's willing to do, I don't think this is the reason she ultimately made the decision.

Others have said "oh she's a crazy targaryon, now we finally see." Dany has been very measured in her actions and thoughtful, even if those actions have been immoral. She doesn't kill people for fun or because she's hearing voices. I don't think her decision to burn the city can be attributed to this.

The death of Messendei definitely adds to her wrath and anger in this moment also, but again, not the root cause I don't think.

Here's my understanding:

The real key to Dany's decision, I believe, lies in the scenes leading up to and after the white walker battle, and then just before the Kings Landing attack.

We see her anxiety and critical insecurity in the northerners not accepting her. This is one of her main conflicts in this season, and is one of the greatest threats to her rule. And not only to her rule, psychologically it is significant that she feels shut out from these people. It matters that she feels incredibly disconnected from everyone around her, even that they are hostile towards her.

In addition, psychologically, the love of her life is having trouble accepting her as his mate/love/companion. This threat of disconnection stands to unravel one of her central relationships.

With the death of Missendei and the betrayal by Varys, Dany's few connections with everyone around her are dwindling and hanging by a thread. Even her must trusted friends and advisors are starting to treat her like a villain, if not just terrified of her. This fear becomes her only interaction with anyone.

And finally, her last hope for connection, when Jon visits her in her room. She is cut off from everyone and everything, Jon is the only person she can hope to keep her from absolute disconnection from love, the world, from being a sensible decision maker, a responsible leader, and an ethical being.

The moment Jon rejects her, the last stand of hope for connection is lost. She has no reason to love anymore, no reason to care for the innocent, no hope for her world.

We as humans live off of human relationships, they are what keep our empathy in balance and our morality in check. Ask psychologists, that the moment a person feels utter disconnection with everyone around them, they are capable of the worst things imaginable.

In Dany's own words, she accepts this:

"...let it be fear then."

All of the other factors above certainly contribute to her decision to do what she did. This seems to be the moment she turned, to me at least.

Let me know what you guys think. There's so much going on with Dany, it was such a crazy episode. I'm so interested in the thinking that goes behind each characters head when they make decisions.