Every other place she liberated eventually treated her like a conquering hero, and she employed terror against those who threatened to reject her. Putting up miles of heads on pikes or crucified masters was intended to terrorize the powerful so they wouldn’t rise up against her. As I recall, her advisors were a bit aghast at that.
But she honestly thought the people of Westeros would hail her as a conquering hero when she ‘broke the wheel’. She was counting on ruling over a supportive people who would learn to love her. But she made a category error - her lack of understanding of Westeros made her think that they were longing for a new leader, but in fact the people of Westeros basically survived by not giving a shit who sat on the iron throne, because in their history it was almost never good. And they weren’t enslaved - just poor. If they were killed by the powerful it was generally as collateral damage in the Game of Thrones. So why should they even care if another ruler shows up?
So now she has realized that these people are different, and that they will never love her. Her one chance for legitimacy with the people was marrying into them, and she planned to do that right from the start. That’s why Dario Neharis was left behind. But now Jon has rejected her, AND he has a better claim to the throne, AND he is truly loved by his people. This essentially leaves no place for the Mother of Dragons and her need to take back the Seven Kingdoms for her family. The only way she sits on the Iron Throne now is through fear - perhaps she convinced herself that after she was in power she’d do good works that would redeem her with the people - tyrants who do despicable things often rationalize it for ‘the greater good’.
Read some more history.
Daeneris being on the brink of sanity has been shown many times. Her advisors pulling her back from the brink has been shown many times. Her inclination to burn people has been shown many times. When she first came to King’s Landing Tyrion had to talk her down from just getting on her dragon and burning villages. That led to a conversation something like this:
Tyrion: Do not do this. You are not the mad king.
Daeneris: I am nothing like my father!
Tyrion: If you go off burning cities and villages, you are exactly like your father.
It worked that time, but back then she trusted Tyrion and still believed that ruling through love was in the cards. And she still had her dragons and all her support structure (Ser Jorah, Missendai, etc). Now she had the same impulse, ut this time she had already lost two dragons, Missendai and Ser Jorah and the man she thought would be her true love and her ticket to power, And we’re supposed to be shocked that this time she gives in to impulses that have been clearly shown throughout the series to be just below the surface?
They’ve screwed up some things in season 8, but anyone who thought that going ‘mad queen’ was not a possibility was not watching the same series I was.