Idk man, it was foreshadowed literally since season one, and the last three seasons have been hammering in that she was one push away from being mad. Maybe the execution was a little poor but it sure wasnt too quick.
Although I can see why a lot of people who have been on the Dany train the whole time can be pissed because the directors have been treating her as heroic despite her committing atrocities since the beginning. If every time Cersei did something evil it was accompanied by a heroic fanfare as she rode a dragon into sunset, youd bet people would think she was a hero too.
She'd just said she would spare people in the bells rang then when the bells ring she snaps and roasts the entire city before going after Cersei. Yes, she was pretty much fated to go mad and the issue isn't that she did, but the issue is that the execution of it was just awful - along with butchering several other character developments at the same time. If they had had something better than the bells she'd acknowledged would be them surrendering then would have been far better, or given her some reason to actually make her snap that doesn't just look like they made it up 2 minutes before shooting. ---