Here, IMO, is Danarys' perspective
why ALL of the smartest characters on the show KNEW that Danarys was going to HAVE to burn Kings Landing,
and why the show was clearly telling us this was going to happen literally the whole time:
The west spurned her family and she was a slave as a result
Baratheon was trying to kill her even as a baby
Varys and the baratheons and lannisters were trying to kill her as a slave and budding queen
She gained power by relying on her personal boldness and dragon power. She conquered the east by being good to the common people, turning them against the elite. She broke the wheel there. It worked and she was loved, as much as any ruler can be.
She came to the west, and tried to play their version of 'the game'
The same game that had gotten literally every single human playing it killed terribly, the good ones, the smart ones, the bad ones, everyone.
When she tried to play 'the game', the lannisters outsmarted her Lannister. The greyjoys outsmarted her greyjoy. The Tarly's flat out told her they knew what a monster Cersi was, but they'd rather burn for Cersi than let 'a foreigner' rule. Half their troops were still standing too. They didn't bend the knee until the dragon threatened them. That says it all.
Then she literally gave up everything to save the world. She met with Cersi and tried to bury the hatchet and join up with her.
Cersi would rather die to a zombie and let the whole world end, along with everyone in in it, than join with Danarys. And Cersi's whole court and people followed her. That says it all.
She tried to show mercy again. Episode 4 can't really be defended, but in concept, Danarys tried yet again to turn the citizens of the west against the most evil of its elite -- Cersi. But Cersi just smirked, killed her best friend, and brought in a willing mass of civilians and mercenaries to hide behind. Danarys knew that mass of civilians was the same group that was cheering and throwing tomatoes at 'Ned Stark the hero' while the most despicable human on the whole show cut his head off. She knew their sentiment, straight from the Tarly's mouths. And from watching them hide in a target city under Cersi, rather than scatter and let her rule.
And guess what? Even her own 'friends' treated her no better. After risking everything, losing most, and saving the world, she's an outcast at her own party.
She tried to befriend Sansa. But Sansa hates her, schemes against her, flat out tells her that the north and its lords will never accept a southern queen, just like Cersi's followers did, and just like the Tarly's did. Basically everyone. Even Arya. Bran offers her no sage advice. The wildlings go home (they know the game is no place for them). Her 'allies' are fighting for Jon.
Still, she even risked it all again, trying to love Jon and rule with him, knowing she would probably die if that happened. (not like Ned Stark and even Jon himself weren't already aerated by their own people) That was her last bid for happiness and to attempt to play the game. And he wouldn't even do that.
So she's alone. 'Let it be fear then.' It's all she has. It's her only card. She's flat out dead otherwise. Dragon. Dothraki. Unsullied. That's it. She has to win with them, completely, or she will die. Period. But the dragon and its ability to destroy whole cities is the trump card. The wheel breaker. She has to use it.
Who are the smartest people ever in the show? Varys? The old Tyrell woman? Cersi?
All three saw with absolute clarity that it was ALL of the west against Danarys, and it was all or nothing. 'You win or you die' as Cersi said.
Varys knows that Danarys will figure out that she HAS to burn Kings Landing, and that has no other choice. He saw her at the party. He saw what happened with the Tarlys, Sansa, Cersi, and the whole bit. He knows that she HAS to burn the city, or die herself. And he knows she's smart enough to figure that out.
He knows it before anyone else. Varys, her own advisor, broke his word to her and is freaking TRYING TO POISON HER IN HER OWN CASTLE, RIGHT AFTER SHE SAVED THE WORLD. He's already written every lord in the west against her.
Lady Tyrell knew it too. As Tyrell clearly saw, Danarys' only chance was to quit listening to 'clever men' and be a dragon. In other words, quit playing the game and burn the whole wheel.
She and Danarys both knew that Danarys will die if she tries to play 'the game of thrones' period.
Even Sansa knows it. Sansa knows that everyone will force Danarys to kill the power of the lords - that means killing the power she and jon just acquired.
The people that don't understand that are clearly bumbling fools, and it shows in their results. Tyrion has ****ed everything up for her, and betrays her twice, all in ignorance of the reality of the 'game.'
Jon is too stupid to rely on for anything. He'll die a second time in Ned Stark fashion. Because he doesn't understand the game.
Danarys can't even give up and go back to Mereen. Cersi and the Greyjoys and the whole west will chase her to the ends of the earth. They always have. They've told her flat out they would. Because of her name and her dragons, the game will kill her no matter where she goes, unless she wins. Her whole life has proven that. So it's win or die.
If you don't want to get eaten up by the game, you have to break the wheel. That means breaking the will and power of ALL of the lords. To do that, she has to break their power over their own people, just like in the east.
But the only way to do that in the west, where the people are unquestionably following these scheming lords, is to KILL EVERYONE IN THAT CITY, and make it clear that anyone following any lord against her will die -- along with their whole family.
That is the only way that the commoners of the west will not rise to fight against her for their lords. If she doesn't do that, she is dead. Period. The whole show has been telling us that.
When she hears the bells ring, she doesn't want to kill everyone, but she knows it is them or her; it's her only chance. She's tried everything else. Those bells were surrendering TO JON. The Lannister army threw down their swords TO JON. Not her. She's not 'all twisted up' because she's crazy. She's twisted up because she hates what she has to do, she's evaluating who these people are, and whether they are worth her dying for them and she decides that they are not.
So its: "burn mother****ers, so that I can live and break the chains of the rest of the west."
I can see some issues with this season, but anyone who thinks 'it doesn't make sense' or 'she just went crazy' or 'it wasn't played out clearly enough' is missing the whole point of the show.
Alas, the problem is 'the game' really isn't just a game. It's fundamental human nature. Even if Danarys wins, peace will only last until she dies or someone isn't afraid of her and takes her or her dragon out. The game only pauses. The most peaceful and prosperous period in history was when Targarions ruled under their dragons. Once the dominant force is gone, the wheel starts spinning faster again while people juggle for power.
I now understand that that's why the whole Night King thing came and went -- that whole plot line just goes to show that even Armageddon itself doesn't stop the wheels of the game, and even the goodwill that comes from overcoming the devil himself doesn't even slow it down.
