Dany killing hundreds of thousands of innocent civilians was not built up properly unless I'm missing something. As far as I'm aware in the past she killed those that were A evil, B against her or C literally fought against her armies and refused to kneel after being defeated. You can argue she did some questionable things.I personally don't think she had to kill the Tarlys but there was reason for it. She was always generally compassionate, particularly towards women, children and average innocent civilians. But what she did to Kings Landing makes her worse than the Mad King. Infact its the most terrible thing that anyone has ever done in GOT IMO.
Her doing something bad due to her looses in order to beat Cercei would make sense. Her deciding to burn hundreds of thousands of innocent civilians after she had defeated all of her enemie's armies doesn't make sense.