Can't say I agree. Executing slave owners and soldiers refusing to surrender and traitors conspiring in your own court is vastly and fundamentally different from burning random women and children.
But when Danaerys ordered the execution of the slaver city elite, she afforded them no trial, executed them by crucifixion - which resulted in the execution of families who owned no slaves, opposed slavery (if you believe the suitor guy. My point is Danaerys couldn't tell afterwords if anyone she had just tortured to death was guilty of anything at all). This would've easily been spotted had Danaerys acted on the advice of her council instead of impulse & intuition. That Danaerys adopted the torture methods of her foes and sought to replace plutocratic oligarchy with feudalistic autocracy was a pretty big running theme throughout the show, where Danaerys was walking the line between fighting to liberate the world, and subjecting the world to the overwhelming awe of her own monarchical splendour. The real tests come when her advisors eventually try to tell Danaerys about the true nature of her father, and also try to teach her the concept of mercy - a concept she had never been shown by anyone from the moment Robert ordered the baby Targs be turned into scrambled eggs. There is not a single moment where she succeeds in learning mercy, right up to the moment she incinerates the defeated Tarlies - who I must remind you, were prisoners of war, not soldiers refusing to surrender. Danaerys did not incinerate them because they were held up in Storm's End daring her to strike - they were unarmed before her soldiers, the justification for execution was that they did not swear obedience to her as the Queen. This is in direct contrast to King Robert who captured traitors, enemies, showed them mercy, took hostages, made marriages and caroused his way to turning enemies into friends - it was clear from the start that Danaerys had the potential to be the last laugh of the Mad King, the one to finally burn King's Landing and achieve it with the form of a dragon.
And so by S8, Robert's fears are realised after the grave. The Mad Queen reaches Westeros with an unknown number of Dothraki screamers laying waste to the Seven Kingdoms. Danaerys is not new to the impulsive elimination of her enemies - so far she has responded to every opposition she has faced with lethal force; the one issue she couldn't deal with violently she left to Daario to deal with. She is a conqueror, not an administrator, nor a diplomat
In the setting of the show, such people can be argued to deserve death. That justice is served.
Dany never shows any heed to the concept of justice, applying laws arbitrarily to her own personal judgement, without obeying any laws herself. She aint' Stannis to put it lightly, and which murderers she pardons and which innocents she executes seems largely
up to the whims of D&D up to what stresses she's been put through. Danaerys is rarely allowed a relaxed moment to think clearly or receive actual criticism from the accompaniment of courtiers who are either overawed, infatuated or in fear of her. Just hear how Danaerys goes from talking about breaking the wheel of tyrants and despots to S8 talking about getting the Seven Kingdoms "back" under her grasp.
The idea of Dany going mad queen is fine in theory, but that kind of degradation in character takes time and investment into showing it that the show just didn't do. Hence my complaint about a lack of showing. An anti-hero taking pleasure in causing pain to those they think deserve it, only to decline into a villain, isn't a new idea and it's been done before.
Yeah the pacing was about as abrupt as me walking into a lamppost xD
I'm defending the idea because anything else is indefensible. Oh wait nah, the music is defensible, the musical score's mostly banging
But who knows, maybe Bran was warging into her and he'll turn out to be the real villain all along.
All this time Bran warged into D&D to warp the script and eliminate all non-Starks
*EDIT
Season 5, the debate between the courtiers is a cool example of what I mean. Barristen was the only courtier who actually told Dany the truth, and was the foremost proponent of mercy over Dany's "justice." Emphasis on Barristen's speech - note how everything Barristen warns of Danaerys, Danaerys
exceeded the mad King.
"Your grace... Your enemies did not lie... He murdered sons in front of their fathers, he burned men alive with wildfire and laughed as they screamed... The Mad King gave his enemies the justice he thought they deserved, and each time it made him feel powerful and right, until the very end."
Sound familiar? I saw the similar parallels as Dany from the books and had my suspicions this would be the outcome from the get go.
Danaerys chooses her "justice," and the form she chooses is every way superior to the Mad King. "I'm not beheading anyone..." Before incinerating sons & their fathers in front of each other
Either way, at this point the show hasn't earned this 'twist', because the writers seem to be more concerned with shock factor instead of the twists like Ned and Robb being twists because they are the logical consequences of the actions of heroic characters that just defy conventional fantasy tropes that would otherwise prevent the characters from facing those consequences.
This is as much an unearned "twist" as Jon's respawn rate is - you know it is inevitable, and you know its execution will be nonsensical despite its necessity
*EDIT
I'm just going to conclude cos I've read more of your post edits and say I agree with you that there wasn't a proper character transition so much as a complete character transplant, even with the foreshadowing acknowledged. Makes Dany shooting down proposals to burninate the countryside earlier a little irrelevant