Dany's story was always going to end like this, and there's enough pieces of it here to see why. I understand the frustration, because ultimately that switch is flipped really abruptly, and it feels jarring. If you really think about it though, it's hard to argue it doesn't make sense, even if one might have preferred a smoother transition, where the viewer is really handheld down her path to madness.
Here's this person who's accomplished all these amazing feats. She went from being little more than a slave, and a whore, to becoming a queen. Going from land to land, city to city, conquering everything in her way. Throughout, she was actually consistently ruthless, and demonstrated kindness/mercy only when she knew there was something to be gained by it (not saying that this didn't happen often, but it was always very much calculated). Especially early on, without the benefit of fully grown dragons, these traits were tools she knew - and was often reminded by those counseling her - she required to build a loyal army. The more power she acquired throughout the show, the more she relied on shows of strength. She still had her ideals of course, and sought to be just in her way, but one definitely got the clear impression that her morals weren't ever going to get in the way of her ambition.
By the time she finally arrives in Westeros, she's become very much impatient. All along the journey she was told she had to do this, that, or the other to prepare for this moment, and then she finally gets here, and it's still not enough. So once again, at the behest of those advising her, she devotes her attention/resources to something else. She helps the North defend the lands against this seemingly all consuming threat of the whitewalkers. In the process, she loses a dragon, as well as one of her most longstanding advisors. That's tough in and of itself. This time however, where she previously found admiration, and loyalty from those she aided, she finds mostly indifference, and actually even contempt. They don't look at her as a savior, but rather….the next problem in their lives. Ungrateful. To make matters worse, the man she'd found love with, is discovered to be her nephew, and the rightful heir to the Iron Throne. At this point, her whole quest, her whole life's mission is basically unravelling before her. Even if/when she wins, there's going to be a challenge to her rule. Whether John wants it or not, he's a symbol to the people of an alternative choice. A familiar choice. Not some foreigner who showed up riding on dragons, and huge armies in tow, but a man of the people. The only peaceful way forward, a marital union, is rejected by him. On top of all this - as if it weren't enough - her actual best (possibly, only real) friend, who's been with her from almost the very beginning, is beheaded in front of her.
So, to say the least, her path to "victory" in Westeros has not been kind. In fact, it's pretty much been a disaster. Plans are coming undone pretty much every episodes, she's down to only one dragon, and she's losing basically everyone she cares about in one way or another. Fast-forward to her hearing the bells of surrender. She's sitting there, on her dragon, taking it all in. Has she really won….? Cersei is surrendering, sure….but what did she win? Taking this city, this throne, has cost her literally everything but the dragon she's sitting on. In this moment, she's emotionally broken. Why should it be so easy for these people? They don't care what she's sacrificed to get here. The efforts she made to keep them safe from an army of zombies. That she's deposing a tyrant. The times she took everyone's advice to seek a peaceful surrender, only to be met with more heartbreak as a result. Why should she just be another in a long line of figureheads who's replacement these people await with indifference? This place is broken. It's not worth saving. It's people aren't worth saving. Cersei is as much a product of them, as they, and their situation are of her. So burn it. Burn it all down. "Break the wheel," if you will.
You can argue with the execution, but I think it works. I'm not saying the slow descent into madness that a lot of people wanted to see wouldn't work, or wouldn't be interesting. I'll even accept that it might have been a smarter move on the part of the writers, to lead people down that road bit by bit, and really spell it out. Make it as on the nose as possible, so people don't feel betrayed when a character they love, and have spent so much time with, is at some moment completely unrecognizable in their actions. Reality though, is that sometimes people just snap, and they do so even at moments that make very little sense to people from the outside. Dany certainly has experienced enough to justify such a snap, and all the character traits that would make her capable of killing countless people, innocents or not, were always there. Whether they were usually held in check by her better judgement, advisors, or an eye ahead to long-term benefits, is frankly irrelevant. She hit a breaking point, and having watched her journey throughout all these years, I personally found it pretty believable.
I do agree that it would have come across better if they'd reversed the order in terms of her destruction of the city and the actual keep, but I can live with it. Flying over it anyway so....might as well torch it on the way, right? lol