I was thinking back to when Daenerys was in Qarth and said that "when her dragons were full grown, she'd take back what was stolen from her and destroy those who had wronged her". That she'd "lay waste to armies and burn cities to the ground".
At the time, I cheered her on. I thought that she was basically talking shit. Yes, she'd kill her enemies. But, there's no way she'd kill innocents. But, when faced with the prospect of burning a city to the ground - including innocents and soldiers who had surrendered, she did, in grief and anger, what she said she would do.
I don't think she's evil or necessarily mad. I think she feels she was driven to cross a line she was willing to cross - if she was angered/grieving/betrayed/etc. enough to do so.
It's the difference between she and Jon, or someone like Ned. There are lines they simply would not cross, no matter what.
Looking at this from 20th century eyes, it's fucked up. But, in medieval times, this was common place in times of war. Additionally, to be absolutely fair to Daenerys, it is Cersei who put these people's lives at risk in the first place, by betraying her pact with the North last season, killing Missandei in front of her, and being so filled with delusion, hubris, and psychopathy, that she felt she would win no matter what. With zero concern for the lives she was risking.
I'm kind of left in a place where I can't really condemn Dany for being who she always was. The Mad King's daughter. But, is she the right person to rule the Seven Kingdoms? What kind of ruler do these people actually want? Someone capable of this kind of carnage who wants the IT so desperately? Someone incapable of crossing the kind of lines Dany crossed tonight who doesn't want to rule? Someone else? IS there anyone else who should rule?
I do think all of this could have been written and crafted better. But, I can't say it's completely out of left field.