So I still cannot grasp how stupid it was that Dany would just go full genocidal maniac after they had won and the city had surrendered.
That's just inexcusably bad writing. Even if you argue that "the signs of Dany's madness were there", that's still way, way too far of a line for her to cross. There's a reason why people find it unbelievable. Dany might have had a darkness brewing in her and all, but to have her turn into a complete cartoon villain, a "literal Hitler", for.... absolutely no reason?
You know what would have made Dany's nuking of King's Landing a bit more believable? A couple of random ideas that I can come up with without even thinking that hard:
- Have the attackers actually struggle a bit more and not have the surrender forthcoming, forcing her hand
- Have Dany approach the Red Keep and see Cersei, still having Missandei hostage, chuck her off the balcony as a last gesture of defiance, making her snap (still not great, I admit, but better than literally nothing)
- Better yet... have Rhaegal actually die in this episode during the battle, making her go apeshit in the heat of the moment (pun intended)
- Have her not burn the entire city, including areas full of civilians that are completely irrelevant to the result of the battle, but at least areas with soldiers, or have her focus on the Red Keep itself and its surrounding areas.
Any of those ideas would have made Dany's heel turn at least more palatable. Sure, you'd have people who'd bitch any way, but at least it'd be... defensible. This isn't. It's just shock for shock's value.
(And that's not even getting with the Jaime nonsense but hey, we saw that coming at least, they've character-assassinated him long ago)