In war, people do terrible things. Full disclosure, I am Israeli, and yes, I have heard people justifying civilian deaths. There were parts of Daenerys' final speech that hit home because I'd heard fragments of it before in a real world context, said by close friends and relatives.
But to turn her into a cartoon villain, a raving genocidal lunatic, was not only extremely unnecessary, cheap, and a gross departure from her character — it was a very shallow critique of war.
George R. R. Martin has never written stories about good guys vs. baddies. With few exceptions (White Walkers), he has always done his best to show us the humanity behind every atrocity, and sought to explore moral quandaries with nuance and complexity. What Daenerys did in episode 5 wasn't human.
In war, people do terrible things, yes, but they do those things because they believe they're necessary. Israel doesn't bomb civilians for the hell of it, they do it because they believe the cost of "enemy" civilian life is worth it if it means taking out one key target, i.e. a terrorist, who's a threat to Israeli civilian life. And this is terrible, and I don't agree, but the thing is there are people who do agree and support it, especially if they have been on the front lines themselves, or have suffered attacks or know someone who has. And whether or not you agree, you can as a minimum empathise with why they hold this belief, the pain and anger that drives them. War is not only terrible, it is complex.
If Daenerys had burned civilians in the Red Keep on her path to Cersei, lost control of her dragon, or unwittingly set off the wildfire that her father had buried under key parts of the city, that would've been a much more compelling story. In the latter case I wouldn't have even minded if she'd chosen to justify/rationalise the destruction of King's Landing after accidentally destroying it rather than allowing herself to feel guilt or be seen as weak for apologising.
I'd detected a certain critique of American jingoism in her speech when she was talking about "liberating" the world, that would have been worth exploring if she had done something more firmly in the darker shades of grey, e.g. uncaringly burning civilians on her path to take out the foreign dictator, Cersei, and then delivered this speech to the traumatised survivors of King's Landing.
Instead they chose to simplify things by effectively drawing on a nefarious moustache for her to twirl and cackle under.
No one is buying this shit.
Also to everyone saying that "mass slaughter of innocents has always been in her character", were we watching the same show?
She conquered a city by catapulting them with the chains of freed men in order to inspire them to be the agents of their own uprising.
She burned down a tent of men in power who were threatening to gang rape her, and thus won the support of everyone standing outside the tent, who was left unharmed.
She executed two enemy soldiers who quite literally asked for it, the leaders of their troop, after offering them a way out which they didn't take, and thus conquered the remaining army without further violence.
She crucified the masters of a slave city as punishment for crucifying little slave children.
She executed traitors which is exactly the same thing Robb Stark, Ned Stark and Jon Snow, our "protagonists" have done, except when she does it it's framed as a doom and gloom act of pure evil. Frankly, this is nothing but a sexist double standard. This is the world they live in. What Ned did was worse; he executed a frightened young man who fled the Wall from certain death in literally the first episode. Where's the outrage over his madness and murderous intent?
She locked up her own dragons, her most powerful weapons, after they accidentally killed one child.
She has never been a person who hurts and tortures innocents for the hell of it. The setting of GoT is a harsh world of dog-eat-dog. She's done what it takes and never more than necessary.
But,
“Necessity is the plea for every infringement of human freedom. It is the argument of tyrants; it is the creed of slaves.” — William Pitt the Younger
Necessity would have made for a much more compelling story for the commitment of atrocities, and a better critique of the horrors of war. The shallow tale they chose to spin of, essentially, a "crazy bitch" who killed people because she was on her period or something, or because she couldn't get laid, or because that's what women do when they get too greedy for power in a man's world, was a farce.