From the beginning, Game of Thrones has flirted with the idea of turning Daenerys Targaryen into a bad guy. After all, in the novels, it is said every time a Targaryen is born, the gods flip a coin as to whether or not they will be mentally stable. Both Daenerys' father Aerys and her brother Viserys landed on the crazy side of the coin. Her other brother, Rhaegar, supposedly did not. Even so, the show has hinted Daenerys could go either way if pushed. Will Dany become the Mad Queen? The universe may have started pushing her to the wrong side of that line. Warning: Spoilers for Game of Thrones Season 8 follow.
Daenerys has always turned hard and brittle when she hasn't had competent counsel. She believes her birthright, her Targaryen heritage, and her dragons give her automatic power to take back Westeros. After all, Aegon had three dragons and he conquered the entire continent in one go when the Targaryens first showed up 300 years ago. (That this is not actually true, and Dorne not submitting until decades after the Targaryens arrived is beside the point.) Daenerys has assumed from the get-go that taking back Westeros would be, above all, easy.
It hasn't been, but for whatever reason, this hasn't changed her perspective. When people don't bend the knee automatically, she turns to dragon fire and intimidation to make them kneel.
There are, or were, two characters who have been by Daenerys' side that have been able to stop this. Jorah Mormont was the first. His quiet, steadying presence had always helped her keep an even keel. When she banished him back in Season 4, the loss of his counsel was obvious almost immediately. Daenerys virtually went off the rails during Season 5. As she pointed out, he should have been her Hand of the Queen. Not just because of their relationship, but because, unlike Tyrion, Jorah could make her listen to reason.
Missandei was the second one who seemed to help keep Daenerys from going sideways. She wasn't just Daenerys' advisor either — she was a friend, a confidant. She was the one who knew first that her boss had a little bit of a crush on Jon Snow. She was the first to figure out the crew would be heading north to help fight the Night King. Missandei had been Daenerys' angel on her shoulder.
And now both are gone. Worse, Daenerys lost Rhaegal on top of it. She already lost Viserion to the Night King. Now she's lost Rhaegal to Cersei and Euron. Daenerys isn't just in mourning for two members of her family that she loved. Daenerys is ready to avenge them, and nothing anyone can say is going to change that.
Who is going to stop her and talk sense? Tyrion? He's a Lannister and she won't listen. Varys? He used to work for Robert Baratheon and is not to be trusted. Jon Snow? That might have been an option before he told her his name was Aegon Targaryen and went from lover to competition for the throne.
There's no one to stop Daenerys from becoming the Mad Queen now.