Although I'm one of those who was of the view that they had laid the groundwork for Daenerys' character in the previous seasons well enough, and after the first two episodes of this season I was pretty sure this sort of conflict was where it was going, I think that it was somewhat rushed (definitely not out of nowhere) in the way it rapidly escalated in episode 4 and 5 from well laid hints to hitting the extreme plot points that needed to be hit before the end of the series. And while I could talk for days about her previous actions and statements suggesting brutal and tyrannical impulses, I can definitely see how the jump from where she was to "deliberately and methodically burn thousands civiilians of King's Landing" is very extreme.
However, if we're going to decide whether something is sufficiently built up or if it's "just bad writing", it seems fair to me to have a go at really analysing what the show did present us with rather than having an initial, surface level reaction of "What the fuck this seems out of character for Daenerys" and sticking to that, dismissing the possibility that a closer look would inform us at all. (Incidentally this is where I part ways from a lot of the criticism of the show elsewhere the internet this season. I feel too much like many people are viewing it in this kind of meta-way where they see it as their story that other people are
wrongly handling, and conversation and criticism mostly is framed in those terms, which makes difficult to have any conversation that treats the story on its own terms).
Some posts here have already made some good points about Daenerys' actions, so I may end up repeating things people already said. But rather than trying to cover it from every angle, I'm going to go back to the Tarlys which I posted about before episode 5. Funnily enough a lot of the points that came from thinking about that situation actually link quite strongly to Daenerys' decision in episode 5.
Part of me feels bad even making a comparison between burning two guys and burning thousands indiscriminately at King's Landing. The two are just so different in scope that it feels like the level of escalation required from one to the other is so extreme that it might seem like you can't draw much from comparing the two. However, regardless of our own sensibilities about the value of individual lives, it can't be ignored that in the context of Game of Thrones, the fate of nobles and their houses are considered extremely important even compared to many lives. When the Lannisters killed all of Ned Stark's guardsmen and servants (dozens or hundreds of people, including innocents), it was bad but there was still a chance for the two sides to make peace. Then they chop off one extra head, and peace is dead, because that was the head of House Stark, and it was symbolically an even greater attack on the North than the slaying of many household guards and servants.
So even though the Tarlys were but two guys, their fate was more meaningful than just two lives within Game Of Thrones because of their status (regardless of our own opinions about it in reality). And what were Daenerys' reasons for burning them? Well, if we take out just a general fondness for righteously burning people, we have:
1. Her
purpose was to make an example of them, to ensure others kneel. When she says "If putting people in chains becomes an option, many will take it" she's connecting their fate to what others will see and how they will act. The purpose of burning them is to ensure others know their fate if they don't kneel.
2. Her
justification was that it was their decision that led to them being burned. "I gave them a choice. They made it." She wasn't just burning helpless prisoners with no agency (in her mind), she gave them a choice and they faced consequences. However, as I made the case earlier, for Dickon especially the choice presented was not a very reasonable one and it wasn't a responsible use of her power to present him with this sadistic choice.
So with the Tarlys we see Daenerys burn people who had surrendered because it makes an example to others not to follow their path, and she sees it as a result of their own choice (regardless of how fair the choice presented actually is). And while it might be quite a leap to go from killing enemy commanders to killing thousands of civilians, in episode 5 Daenerys' dialogue shows her own twisted logic that brings the situation in King's Landing to the point where she can use those same principles to make her decision.
"That's what she thinks our mercy is: weakness. But she's wrong. Mercy is our strength. Our mercy towards future generations who will never again be held hostage by a tyrant."
You know it's going to be bad once "mercy towards future generations" comes into it. But this shows how the fate of the civilians of King's Landing becomes about more than just the battle for the city, and why Daenerys burns them even after it appears she has already won that battle. Just as the fate of the Tarlys was about the example set to others rather than the immediate need to deal with them, so was the fate of King's Landing. And it's not just to the rest of Westeros outside of King's Landing - it's to "future generations" throughout Westeros.
But, in the case of the Tarlys the example to others was to "kneel or burn". Right or wrong you can at least see what the message is. What's the message in burning civilians of King's Landing after breaking through the opposing army's defences? Well, this is revealed by Daenerys' dialogue as well:
TYRION: The people who live there, they're not your enemies. They're innocents, like the ones you liberated in Meereen.
DAENERYS: In Meereen, the slaves turned on the masters and liberated the city themselves the moment I arrived.
As far as Daenerys sees it, not only did Cersei and the opposing army make their choice, but the
people of King's Landing made their choice as well by not overthrowing Cersei the moment Daenerys showed up. When Tyrion tells her the people are hostages, she even says "Whose fault is that? Mine?" I think that the quote above shows that not only does she not see it as her fault, she sees it as the people's fault as much as their ruler's. Of course, the "choice" that the people of King's Landing made was between accepting the rule of their current oppressor or rising up in violent rebellion that would get many of them killed. The audience probably sympathises with unarmed civilians not rising up and doesn't hold them responsible, but as far as Daenerys is concerned they chose their fate just like Dickon Tarly did. And the message to every individual in Westeros, present and future, is that if you even tacitly support any other ruler by not violently rising up against them, you're going to burn. That's a message she hopes will serve well when anyone starts talking about someone else having a competing claim.
When she decided to "Let it be fear", she gave up on trying to secure her rule by other means. It might seem that the burning of civilians after already winning in the field and gaining the surrender of the city didn't serve that purpose as that should already cause enough fear. But considering the reasons behind Daenerys burning of the Taryls (after they were defeated in the field and had surrendered) and Daenerys' dialogue in episode 5 we can see how the act of torching King's Landing follows on from it in terms of Daenerys' mindset, even if it's on a much, much bigger scale.