The best (completely out of continuity) explanation for Dany's abrupt heel turn happens in the "previously on Game of Thrones" section for this episode, with a dozen voices in her Dany's head as she watches Cersei behead her friend, including the Queen of Thorns ("BE a dragon.") and Viserys ("You don't want to wake the dragon, do you?"). If you haven't seen it, you should look it up.
All that being said, Dany's descent into war crimes villainy, even though it's heavily foreshadowed throughout her entire journey, was rushed and unearned. I thought for a long moment she was going to simply fly Drogon to the Red Keep and burn Cersei/Qyburn/FrankenMountain alive - and that totally WOULD have been an in-character, earned moment. But the plot had other ideas, including Cleaganebowl, that I will get to later.
The first death is also the most heartbreaking, especially since, by the end of the episode, we learn that Varys was right. "I hope I deserve this, I truly do." He didn't. So he dies for advocating on behalf of the realm. Goodbye my friend, you deserved better. Note - he died by fire on the same beach where we first saw Melisandra and Stannis - when they were burning the Seven gods in effigy.
I didn't like Tyrion betraying his friend, especially when it turns out Tyrion was horribly wrong. Tyrion used to be portrayed as smart. How many times has he given terrible advice to Dany on strategy? And now he steadfastly stays loyal to the Mad Queen despite being abjectly afraid of her and afraid of what she might do.
On to Jamie, whose whole redemption arc was betrayed last week in his speech to Brienne ("Cersei is hateful, but so am I."). His fight with Euron wasn't strictly necessary - Euron should have died by dragon fire as retaliation for the death of Dany's child, but instead he dies at Jamie's (bad) hand, because why? He kept reminding Jamie that Jamie sleeps with his sister? Dramatically the fight served no purpose. And somehow Jamie takes two stab wounds to his core and can still find the strength to kill Euron and make it to Cersei? Either his leather armor is amazing, or his plot armor is.
On to Tyrion. Hated his betrayal of Varys, which turned out to be a betrayal of Jon, of the realm, and of himself. But I loved his last moment with Jamie, and I was glad these two characters had the chance to say goodbye. Tyrion's (too late) epiphany moment - that his life vs. tens of thousands of KL citizens = impending dead dwarf - was great, even if he should have turned on her the moment she incinerated the Tarlys. Tyrion also becomes the point of view character during the surrender of the city and Dany's decision to burn it anyway. We, as viewers, have cheered for Tyrion and it turns out we misplaced our trust in him, since his actions since becoming her Hand have made this slaughter possible.
Jon continues his uselessness during yet another battle. He is unable to control his own men as they turn into rapists and murderers of innocent people. (Side note - have we been wrong to cheer for the North all along? These people are monsters too.) He basically spends the episode with his mouth open in bewilderment, which is more or less his default state. If he becomes the big hero of this story it will be completely out of character. He doesn't deserve to be King. Also - his utter devotion to someone, just because he "bent the knee" and swore loyalty to her turns out to make him look weak and ineffectual. Like Tyrion, he took too long to see how horrible the Mad Queen had become. #Sansawasright
Grey Worm - completely in character throughout the episode. He wants his pound of flesh and is willing to take it. These people mean nothing to him, to the Unsullied, or to the Dothraki. He gets his moment taking out both the commander of the Golden Company (who doesn't even get a word of dialogue) and the commander of the Lannister army. That they didn't honor the surrender shows the type of leader Dany, supported by her throng of foreign invaders, will be. We all thought that "breaking the wheel" perhaps meant democracy, when in fact she is burning it to the ground. All Hail the Queen of the Ashes.
Cleganebowl. I really liked it, actually. The directors here did a great job of making me think that Sandor may actually die (when Gregor starting digging at his eyes). I did like the ending too. He couldn't kill the already dead - so he sacrifices himself to rid the world of the unstoppable evil that his brother had become. Also fitting that he died by fire, which had always been his greatest fear. Bonus points for Qyburn's hilarious death.
I didn't like Cersei's end. She barely got a word of dialogue in this entire episode. But kudos to Lena for facial acting, as she slowly goes from smugness to denial to a horrifying realization of how outclassed she always was. Lena's acting was also great when Cersei found Jamie. But going out crying was not the way I would have got rid of the series' primary villain. I guess it's symbolic that she dies with Kings Landing literally coming down on her head.
Arya is a point of view character for the horror felt by ordinary people during the horrifying dragon attack. She survives more by chance than by design (plot armor again, I know). But the sequence does a great job of showing how the people of Kings Landing must have felt. The Dothraki indiscriminately killing in the street made it feel like everyone - including the viewers - have made horrible mistakes in supporting Dany's claim. And it was tragic that Arya, the character who saved all of Westros from the Night King, nearly died an anonymous death as the dragon swooped again and again. One thing's for sure at the end of it all - Arya has a new name for her list.
Now is a good time to say that despite the story contrivances, this episode is beautifully shot, directed and scored. From a technical standpoint, this episode is the best of a great series.
After this episode I went back and watched Dany's vision in the House of the Undying. She walks into the ruined throne room of the Iron Throne, treading on what is either snow or ash. She almost touches the throne but doesn't quite get there. Her next scene fades to white as she goes through a white gate (which we thought was the Wall). Beyond the gate she gets reunited with Khal Drogo and the son they had together. So I guess we already know how Dany goes out. Just as she is about to mount the throne she will die and fade to white. The likely candidate is Arya, the ultimate hero of this story. Jon was just a red herring, it seems (and I hope). I suspect Arya will assassinate Grey Worm, take his face, and kill Dany just as she sits on the Iron Throne for the first time.