Too many viewers are confusing the brevity on-screen with the actual timeline. She didn't go mad overnight. She's been simmering through all of her losses for weeks/months.
She lost two dragons. Jorah died at her feet. She fell in love with Jon then lost him. Missandei died in the most brutal way just in front of her, and not for any reason—just a massive ** you from her only real rival. And most of all, the thing she had been gearing up for for years—to finally rule the lands by birthright—was seized from her. She still wants that, and still can justify taking control back, 'by any means necessary,' if she still believes the world will still benefit from it. She basically said that even if there is massive carnage now, it's for the generations later.
Viewers rip the show for doing the obvious and then they rip it for not doing the obvious. I guess most thought Arya was going to kill Cersei. But, the torching of the city created a greater rationale for Arya. That's something. The Clergane Bowl was more epic than Obi Wan/Vader. That's something. The White Horse was beautiful. But, the millenials had to have their millenial fun and meme it out: "Bran ordered Arya an Uber." . It's a story.
We've been treated to, what, five epic battles now? And all of them have been bigger and better than anything ever in the history of tv or cinema. And people whine. The language and dialog is better than anything since The Lion in Winter (1968, an Academy Award winner), and they do it 6-12 times a year, while covering vastly more characters, plotlines, history, and scope.
Like anyone else, i have my lists of expectations and predictions. I've been massively wrong on most, and i do believe some of them would have been better than what actualized. But, all of that ignores what does happen on screen, and it is still an insanely spectacular program. And the filmmaking is so good, almost effortlessly so, that people take it for granted. You can look at any five seconds of screen time, break it down into what was required to accomplish it production-wise, and you should marvel at it.