Let's first address the elephant in the room: the Mad Queen plot-line, which many fans hoped would turn out to be a red-herring after the last episode, turned out to be true, and it played out like a nightmare in the penultimate episode (episode 5), titled The Bells, of the Game of Thrones season 8. 

To see the biggest fall from grace of one of the most powerful characters you rooted for through 7 seasons, can be a very hard thing. But definitely a raging madness cannot be put past Daenerys Targaryen's character, and very few would deny that. How the creators got to this point is surely worth debating.