Daenaerys going mad is a great plot point but terribly executed, like most of it since they moved on from the books. Quality has gone miles downhill.
Ultmately this is obviously a poor attempt to shoehorn in the end to the books without taking the required steps to get there. Going mad and burning an entire city wouldn't be massively out of character for book Dany, by the end of the fifth one I she really isn't far off that. I think she comes across as much more of a hero in the show which is the main problem, in the books I never actually thought she came across as someone who'd make a good ruler. And she's a Targaryen, if anyone was actually expecting her to be the ultimate hero they were hopelessly fooled by the cult around her character (which is really the point, present her as some kind of messianic leader who's going to right wrongs, protect the innocent etc but when it comes down to it she's just like everyone else). It's a brilliant twist (if it's even that, heavily foreshadowed looking back, especially in the books) and sums up the entire point of the series, there aren't any good/bad guys and the 'honorable' people like Ned/Robb die early on. The whole point is to have us cheering her on for all the books/show and then show us that we're wrong and she's just as bad/worse as everyone else when it comes down to it. It's a brilliant ending, but the show couldn't pull it off without having every plot beat handed to them without Martin and when that was taken away it was shown that they couldn't keep it going. Instead we got this rushed, half-baked descent into madness that wasn't at all convincing.
Honestly my biggest gripe, even above all of that, is that all the smart characters have ended up being idiots, on the basis of this season Varys and Tyrion come across as idiots. The writing since Season 5 has just been really, really poor.
As someone who loves the books (except for A Feast for Crows which is s***) it ****** me off to see a great story ruined so spectacularly. It seems fairly clear that GRRM isn't a fan of where they went with it though, so hopefully that spurs him on to finishing the books, which I had half accepted I won't see (at least not written by him).
Anyway binge-watching this season was meant to be my coping mechanism for losing to Saracens and then they go and pull this. Can't catch a break right now.