As a point of comparison, Walter White's flaws such as pride and willingness to kill if pushed were telegraphed early, but it still took 4 seasons to go from the "Chemistry teacher with a dark streak" to the Heisenberg that was happy to feed a kid ricin to achieve his goals.
Another brilliant example of Breaking Bad's strong writing is Walt doing something completely and utterly stupid, but in character: when Gael was killed and Hank thought he was Heisenberg, Walt couldn't keep his trap shut and hinted that Gael was just an imitator, because he was so stubborn and proud that he couldn't allow for anyone else to get credit for "Heisenberg". It was something completely stupid (I was almost yelling at the screen "shut up! shut up you idiot, shut up!", but absoutely in character for Walter White.
The whole Dany thing, instead.... the more I think of it the more it leaves me puzzled.
I accept her being ruthless and having her selective violence becoming less selective, but the events of the penultimate episode are weird.
Dany said something about "Cersei thinks our mercy is a weakness", implying that she might not care for the people in the Red Keep.
Varys says as much to Jon ("We know what she's about to do"), so Tyrion begs with her to give a chance of surrender to the city, the infamous ringing of the bells.
Tyrion then talks to Jaimie and he looks damn sure that the city will fall - "No, Cersei will lose", "There won't be an Iron Fleet much longer", how is he so sure? Did Dany told him her plan? if so, that would mean that Dany said "I'm gonna destroy the Iron Fleet attacking from above, then I'm gonna take out all the scorpions and open the gates for my army", which was already a victory plan with minimal civilian casualties, did she add "then I guess I'm gonna burn them all"? or Tyrion just figured that the dragon was invicible anyway, something that for the record negates the whole "Oh no, Euron killed a dragon, there are scorpions, Dany can't use freely Drogon anymore!" thing?
We then come to the battle: Dany pulls out her perfect plan, in 10 minutes or so destroys the fleet and the walls, the city falls, and then she just snaps, with the official explanation coming from the authors being "she saw the Red Keep and she thought of all her family has lost". She then goes street to street to methodically destroy and burn everything, giving all the time in the world to Cersei to escape, which she almost did. The equivalent, in case the White House is occupied by a tyrant, of needlessly carpet bombing the city of Washington while the White House sits there alone in a park.
She could have flown to the Red Keep and burn it regardless of the civilians inside, with maybe some wildfire caches exploding accidentally and creating more damage - and then when confronted she could have angrily said she didn't care, proving she was losing her compassion. But apparently some nuance wasn't enough so she had to needlessly destroy the city AND have the dragon behind her to give the impression of her being a demon with wings AND a huge red Targaryen banner to call back nazism AND her speech about conquering the entire world like she never, ever, ever said she'd want to do.