I actually think Dany's breakdown is one of the more believable character developments. Going beyond the fact that she's a pure blood Targaryan, who are unstable and violent due to inbreeding, throughout the entire show she has been living under the singular idea that she has divine purpose--that she accomplished the impossible by becoming mother of dragons, that she has rightful claim to the throne, that she is allowed to take extreme measures to defeat her enemies because it's for the good of the kingdom which she believes only she herself can bring. She has always had a propensity for violence, but it was always justified by this belief that her actions were governed by divine right, like when she freed the slaves in Mereen who murdered all the masters. Furthermore, she has always preferred capital punishment as opposed to imprisonment, and only two people could ever talk her out of going this violent route: not her royal advisors Tyrion and Varys, but rather her two closest friends Jorah and Missandei.
So, flash forward to Season 8, and we see her whole belief and support system that she has held for 8 seasons fall apart. As it turns out, she has no legitimate claim to the throne because of Jon; she isn't destined to rule and save Westeros. While trying to cope with this, she is constantly confronted with the reality that Jon would be a truly benevolent ruler who commands loyalty through respect alone, which is what she wished for herself but can never have because the people of Westeros fear her. She formed these goals and ideals to help herself move past her own abuse and subjugation, and now she knows that her life has essentially been a lie. This makes it easier for her to give into her more violent impulses, and the only two people who would be able to talk her down--Jorah and Missandei--are dead.
So, cue the battle for King's Landing. This is a battle that Tyrion--someone who Dany sees as a failure of an advisor--has been saying doesn't even need to happen. Tyrion has held to the point that by asserting minimal military dominance, Cersei's people would lay down their arms and show their disloyalty to her. He held that they would ring the bells at the first sign of an unwinnable fight, which Dany simply did not believe because she has lost faith in Tyrion. And, it turns out, he was right. So, she has lost her claim to the throne, her destiny, her lover (since Jon won't knowingly sleep with his aunt), the only people she considers friends, the people of the kingdom fear her, and her life's goal is overall a sham. There was nothing hold back her violent side anymore, and all it took was Tyrion's "see, I told you so" victory of King's Landing ringing the bells to cause her to snap.
Now, do I think all of the other character developments this season are garbage? Abso-fucking-lutely.
