Daenerys lost her mind and killed nearly everyone.

It looked like Dany was just going to fly over to the Red Keep and blast the tower Cersei was in (which I believe is how Harrenhall fell), but... yeah.   They really didn't clue us into her thought process. 

Maybe if they'd had her starting to hear voices throughout the season, as happened to her father towards the end.  Or maybe if the surrendering Lannister soldiers were kneeling to Jon while she watched.  But as it stands, it's a completely unearned moment in a series that usually puts the leg work in.

That push over the edge is all that's missing, though.  Looking back, it's not a shock that Dany went berserk.  Didn't she pretty much want to handle all of her conquests this way, until Jorah, Barristan, and/or Tyrion talked her out of it?  And of those, two are dead and the last can no longer be trusted.

Almost as stupid as Dany burning everyone being unearned was the character assassination of Jaime. Like how he says he doesn't care for the people of King's Landing when that was the reason he became the Kingslayer in the first place. And in the end, nope, he still wants to fuck his sister all the way to his death. That duel with Show!Euron was all-time stupid, too.

I was puzzled by that moment with her and Drogon on the rooftop, and how it really didn't come off like a person snapping or going mad.  It just didn't work.

But that's because she wasn't snapping in that moment, and there was nothing spontaneous about what she did.  Her breaking point was when Missandei was killed.  And after that, Dany made the cold, calculated choice to torch King's Landing.  To make an example.  After Tyrion, Varys, and Jons' apparent betrayal, she made one last plea to Jon to pull her back from the brink, but he rebuffed her.  "Alright, then.  Let it be fear."

Tyrion begs her to listen for the bells, but she gives him the brush-off.  Just a knowing nod to Grey Worm, the only person left that she really trusts, and also, I'm guessing, the only person she told about what she's planning.

Dany lands on that rooftop, waiting to make sure the Lannister troops don't surrender, and the bells don't sound, in the interest of plausible deniability.   Hey, she gave Cersei a shot.   But when the bells do sound, she chooses to just grit her teeth and carry on as planned.

This wasn't Anakin Skywalker going berserk on a village of Tusken Raiders.  This was Darth Vader calmly marching with a legion of clones into the Jedi Temple, to massacre everyone inside.