Dany witnessing her vision of the throne room made real sort of rubs salt into the wound. If she'd seen another vision coming true beforehand, it would have provided an excellent explanation for her going over the edge.
That's what the script was desperately trying to *tell* us, through the mouths of Varys etc - she'd become dangerous because she believed too much in her destiny. It's dreadfully disappointing that they didn't find anyway to SHOW it. Worse than that, what they chose to SHOW us about her inner state - trembling lips - took us in a completely different direction, and led us to believe it was about Cersei making her angry. And then there's the implications of the story, which was clearly pointing to her being affected by the death of her dragon and feeling isolated. Finally, D&D throw yet another spanner into the works with their comments about her forgetting about the Iron Fleet? What, so maybe she's really angry with herself about that?
It's a mess. All going off in different directions. Smacks of them not wanting to make any commitment or decision. They needed to pick a couple of believable and compelling explanations of/for her descent into madness, and then portray them using ALL the elements - the plot, the story, the dialogue, the action, and the performances - in unison.