Tyrion to Jon:
"When she murdered the slavers or Astapor, no one but the slavers complained. After all, they were evil men. When she crucified hundreds of Meereenese nobles, who could argue? They were evil men. The Dothraki Kahls she burned alive. They would have done worse to her. Everywhere she goes, evil men die and we cheer her for it. And she grows more powerful and more sure that she is good and right. She believes her destiny is to build a better world for everyone. If you believed that...truly believed that...wouldn't you kill whomever stood between you and paradise?"
Tyrion literally explained it to Jon and the entire viewing audience. It wasn't so much a "turn" of her personality, but a change in context that outed her as a self-absorbed tyrant bolstered by claims of "destiny" and a dragon or three.
Daenerys used the phrase "break the wheel" a lot, but the reality is that she was a slave to it wherever she traveled. In fact, her claim to the Iron Throne itself only holds if "the wheel" is in one piece and spinning firmly in place.
BTW, in the books, during the sacking of Astapor, Daenerys orders that only children younger than 12 be spared. I'm sure the Unsullied and Dothraki forces weren't checking birth certificates, so that probably meant that most children 10 years old or or older were slaughtered. The show glossed that over by changing it to "harm no child".