And there are way more instances of Daenerys acting in a good, noble, and pure way.
Yes, in season 3 she sacks the city of Astapor. But, in that same season, she also frees the Unsullied, gives them the choice to follow her or not, proclaims that she will never spill the blood of innocents, frees the people of Yunkai, and is visibly happy that she is loved by the people, etc.
But she said once that she'll burn cities to the ground so now it totally makes sense that she goes from "protector of the world" to "destroyer of the world" in two episodes. Because this is literally what happened. Episode 3: Daenerys risks her life to save the world from the White Walkers. Episode 5: Daenerys, ugh, "goes mad".
I'm so happy that GoT has disappeared from public memory.