There was plenty of development to have her destroy the Red Keep, and the horrifying thing would've been that she did so even though Cersei had filled it with innocent people. There was plenty of development to have her destroy the Lannister soldiers in the streets, leaving 0 quarter to the surrendering army.
There was no development to have her burn every inch of King's Landing and destroy all the people who lived there. There was no motive (Cersei pissed her off, but none of those people even liked Cersei), there was no benefit (none of those people would've cared who the Monarch was), there was no development leading to her personality cracking (your advisers deciding you're crazy and your nephew not kissing you right now just isn't enough to make you commit genocide).
That the ending the book is shooting for, almost certainly. D&D didn't earn it. Since they didn't earn it, they'd have been better off going with something else.