Disclaimer: This is going based off show canon and not book canon.
Here is all the previous evidence of her poor decision making and themes in Dany's arc:
Barristan gives Dany some insight in Meereen that is quite applicable to Dany. "The Mad King gave his enemies the justice he deserved, and each time it made him feel powerful and right, until the very end"
- In Meereen, Dany crucifies the Nobles indiscriminately without a trial, because she saw children crucified on her way to Meereen. While that is definitely sad, her actions are very impulsive and to a certain extent, cruel. Hizdahr Zo Loraq has to beg Daenerys to let his father's corpse off the cross despite the fact that he had no part in crucifying slaves. Not only is the fact that she lets the corpses rot on the cross impulsive, its insanely cruel. Cruelty should not breed cruelty or it will become an endless cycle. Crucifixion is an extremely painful way to die and she chose it willingly in order to make a point or show of force. This is a recurring theme that Dany will commit acts of extreme cruelty because she believes it is the right thing to do.
- When Hizdahr goes to Yunkai, the masters of Yunkai agree to her terms except that they believe the fighting pits should remain open. Not too unreasonable..but Daenerys has always wanted complete authority, she rejects terms on the basis that she's a Queen, not a politician. Sound a bit too powerhungry to want unquestionable agreement from a city you conquered?
- When Barristan is killed by the sons of the harpy, she immediately once again indiscriminately arrests the Nobles of Meereen. You'd think she learned her lesson about rushing to judgment but no. On top of that, she forces them into a dungeon to confront her dragons. Herself admitting that she has no idea who's innocent or guilty, she arbitrarily executes a master, for all we know could be innocent, by burning him alive. Wow, im sensing a pattern here. I'd like to point out it would be no trouble to her to have an executioner kill the master for her, and give him a more painless death but no, she chooses Dragon because Cruelty sends a message. She is not to be messed with.
- When does she have a change of heart? When Hizdahr gets on his knees and begs for his life. That's what she wanted and she achieved her goal. Total and complete surrender. Not to mention Hizdahr never even says he intends to marry her, she in a way forces him to marry her. You might argue, but he could've said no... Totally, he can say no to the queen who just burned a random person alive in a show of force and who needs Hizdahr for political purposes to bring peace to Meereen.
*Brief tangent: Look at it from Hizdahr's POV. Dany arrives in Meereen, conquers the city and then executes his father for a crime he didn't commit. She then arrests anyone associated with his class, and arbitrarily executes a random person, and nearly him. Then when she finally realizes she can't rule without the support of at least some of his class, she forces him to marry her. Sound familiar? Joffrey becomes king, executes Ned, abuses all those associated with the Starks, and then when Robb wages war publicly and only keeps Sansa as his wife so that she can write to Robb to pledge fealty to Joffrey. Hizdahr is also stabbed during the attack in the pit, proving that he wasn't involved with the Harpy.
5) Tyrion's arrival in Meereen highlights her righteousness and belief in destiny. Im not going to stop the wheel, im going to break the wheel. Now some might say oh shes doing that for the people. Something something crushing those at the bottom. But when she asks Tyrion for help what does she ask for? How to help the people? No. To get what she wants. The Iron Throne. Her first thought is not about helping the people, its about getting the throne.
*Some might argue that she helps the North defeat the Night King, but in my opinion this can be explained pretty easily. 1) She loves Jon (who at this point is no threat to her rule and is actually an ally) and 2) She's seen the threat firsthand and it's pretty clear that if Jon and the North lose Winterfell, the Dead will defeat the entire realm.
6) When Meereen is attacked by the slavers, her first impulse is to burn their cities to the dirt. Only Tyrions counsel and comparisons to the Mad King stop her from embarking on her quest.
7) When Jon Snow arrives in Dragonstone, she demands his immediate surrender. She reminds him of the pledge his ancestor made to Aegon the Conqueror. She offers him nothing in return but when he refuses on the rightful ground that the pledge between Lord and King was broken when Dany's father burned his family members alive, she declares her father evil BUT she doesn't acknowledge that the pledge was broken and instead declares Jon in "Open Rebellion". Once again, demanding complete submission for absolutely nothing. She feels entitled to Jons loyalty simply on the basis of her ancestry without acknowledging that the Monarch is supposed to protect their vassals in exchange for fealty and that fealty is not a right. Im reminded of Book Varys's speech about Tommen thinking Kingship is a right and not a duty.
8) When the Tyrells are killed and she destroys the Lannister army in the Field of Fire, she captures the Tarlys. She has choices, prison, yada yada. Tyrion gives her advice to imprison them till they bend the knee, but that wouldnt be complete submission and it would never satisfy her. So she does what she thinks is right, and burns them alive. Once again, using cruelty as a show of force to make a point. Some like to argue that burning alive isn't cruel...and lol. In the context of the show, Tyrion clearly sees burning as WORSE than beheading, because when Dany mentions that shes not beheading anyone, Tyrion is audibly desperate for her not to do so.
9) She forces Jon to give up his claim so she can be queen...but Wait...what happened to the rule of succession? She's seen evidence of the character of person Jon is and yet she forces him to keep his lineage a secret in order to usurp his crown. "What happens when they demand you press your claim and take what is mine?" UM... what... he is the heir to the Iron Throne. The Son of the Crown Prince in every rule of succession comes before the Sister of the Crown Prince.. In What world is the throne Dany's if Jon is who he says he is.. How is that not powerhungry of her to step over him to rule? Destiny? If so Varys put it best, Destiny is one of the easiest ways to justify cruelty.
Dany believing that she is the savior of men is simply a way for her to justify seeking complete authority whereever she goes. Her disregard of Jons claim unveils this pretty clearly and complete's her character arc, and I think her arc is pretty damn amazing. Tyrants don't become powerful unless they have some support from the people. They don't start off with total annihilation or they'd never get the support needed to start. No, they start off doing things that some people love. A lot of tyrants appear better than what came before (Castro and Bautista for ex), but they're not good. The fact that a lot of people are rooting for Daenerys is great and a lesson for us all, many of us don't realize and may even support a Tyrant until they're too powerful to be stopped. It subverts the Chosen One prophecy and shows us exactly what's wrong with that type of thinking..which is why I believe this will be in the books as well. (theres even more evidence for this in the books)
Thats why I loved Varys in the last episode bc he sees it ahead of time and really shows how smart a player he is.
