Here's a different take, which I thought was quite interesting and compelling https://twitter.com/DSilvermint/status/1125856091261136896
But like a kid who partied too hard senior year and found out his final project is due in a week, they seem to have woken up one day and realized they'd moved way, way too far from the source material, and then just jerked the wheel to steer it back in a straight line towards the intended book ending. Characters who've developed differently in the show than in the books? Whiplashed back to the book versions. The Night King taken down in a single episode, with no real understanding of what he was trying to do, or what any of it really meant. Dany destroying all of King's Landing, but instead of a culmination of increasingly morally grey choices as she takes the throne from a loved leader, she's murdering civilians who don't even like Cersei because a bunch of dudes kept saying she was crazy (or something).
Frankly I'd have preferred they just made up some hollywood ending that vaguely fit the direction they'd brought the show, instead of giving us all whiplash.
From, like, season 1 on Dany is established to be explosively vindictive, especially to perceived betrayal by those who are either personally trusted or whom she feels owe her duty, as well as desperate for (and violence tempered only by) receiving love on both an individual and mass level.
And those factors are directly the source of the violence she turns to in S8E5. The direct lead up has been decidedly unsubtle and started before S8. It wasn't a culmination of increasingly morally gray choices, it was a simple continuation of a pattern of behavior that's been constant the whole show combined with a pattern of increasing erosion of the factors that were previously established as mitigators combined with an increasing accumulation of factors previously established as triggers.
That it was headed for something like this has been evident for some time, that it was likely to erupt at the particular time it did against the particular target was clear in S8E4 with Missandei’s killing even before they laid on more triggers in the part of S8E5 preceding the break.
You can only foreshadow and drop hints about a character changes if you do those change away from the action (the protagonist that will change is away from the limited narrator), or if the spectator do not know the character that well (he is one of the antagonist).
Let's compare to other "masterpieces" with a lot of public love. Does the character change feel as justified as Darth Vader's one in Return of the Jedi? Or even as Severus Rogue (Harry potter) change? Those change are not really well written imho (a bit too rushed for the 1rst one, a bit forced for the second, but that's only my opinion). If the character change in GOT is not at least on this level, i would understand why people are disapointed.
The error GoT have done for me is from way back. The first few seasons used realistic plot, few plothole, believable and clever characters, pretty realistic choices and political stuff. My suspension of credulity bar was set pretty low. Season 4 was weird regarding time and distance, but still mostly believable (and the dialogs were really good too). But S05 broke my suspension of credulity almost every 10 minutes and that made the show a bit boring (imo).
That's the issue for GoT: they did set the bar too high at first, then couldn't reach the same level again.
In fact, I'd argue that a distinguishing feature of GOT as a series (and ASOIAF, the book series) is that there is remarkably little character development in the narrative, and even further that a major theme of the narrative itself is that binary positive and negative character assessments tend to be made on the basis of circumstances when the same character in other circumstances would be assessed very differently, and that changes of circumstances are easily (and dangerously) mistaken for changes of character.
There was plenty of development to have her destroy the Red Keep, and the horrifying thing would've been that she did so even though Cersei had filled it with innocent people. There was plenty of development to have her destroy the Lannister soldiers in the streets, leaving 0 quarter to the surrendering army.
There was no development to have her burn every inch of King's Landing and destroy all the people who lived there. There was no motive (Cersei pissed her off, but none of those people even liked Cersei), there was no benefit (none of those people would've cared who the Monarch was), there was no development leading to her personality cracking (your advisers deciding you're crazy and your nephew not kissing you right now just isn't enough to make you commit genocide).
That the ending the book is shooting for, almost certainly. D&D didn't earn it. Since they didn't earn it, they'd have been better off going with something else.
And people who by situation of birth are in a group whose way of of life conflicts with her moral preferences, even if they've never personally wronged her or owed her a duty of allegiance, c.f., Astapor.
> Yet every time the thought of killing innocents comes up, she's gone extremely out of her way to avoid it.
To the extent that’s even arguably true—at least, after Mirri Maz Duur in S1 (the negative results of which are the impetus for the one Stark example of Dany’s character development)—it was directly tied to a belief that she was loved (either personally or as a leader by the masses or, most often, both) and that acting otherwise would undermine that—a factor which very much was not at play in the attack on King's Landing.
And she's made very clear since very early on (though it's clear her practical expectation that this duty would be fulfilled has evolved over time) that she feels the masses of Westeros (and therefore those of King's Landing) do owe her a duty of loyalty that they betray by obedience to anyone else. It also made clear that her relative restraint was due to the restraining influence of a particular set of advisers and allies, all of whom were either dead or seen as betrayers (the latter—and in one case also the former as a direct result—in a couple key cases in part largely because their confidence in their ability to continue to restrain her impulses was slipping for reasons clearly shown in the narrative) by the time of the attack on King's Landing.
>What big moments did they want to deliver? Where should the characters end up? What did they think we, the audience, wanted to see on screen before the show came to an end? It was a Game of Thrones bucket list. /18
This tweet from that set sorta highlights it. Because choosing to build a timeline working backwards is different than how they approached filling up that timeline.
> A show that had been about the weight of the past became about the spectacle of the present. Characters with incredible depth and agency - all the more rope with which to hang themselves - became pieces on a giant war map.