Danerys going crazy should have been set up YEARS in advance. She spends the first 7 seasons doing everything she can to avoid needless suffering and talking about how she's going to "break the wheel."
Then, over the course of only 3-4 episodes in Season 8, D & D flip her switch from "good" to "evil" like the Krusty the Clown doll and she slaughters an entire city. This is not "character growth." This contradicts everything we've been shown about her character.