Her “fire and blood” tendencies had been well seeded. She makes many threats to burn down Astapor, Yunkai, and Meereen. I mean, in the previous episode she said she would burn KL down and was lukewarm when Tyrion told her not to. When she’s on the dragon and the bells are ringing, she makes a self-revelation that she is alone. The one thing she always wanted is in her grasp, but Westeros doesn’t feel like home (which they also explicitly stated multiple times). Then she makes a choice to burn down the city. This turn from characters happens literally all the time, but I think maybe people didn’t like it (other than being Dany stans) is because normally the turn is positive instead of negative. The negative turn I can think of is Anakin to Darth Vader, when he has the revelation that the Sith and Jedi aren’t so different and he has the choice to kill the Chancellor or save him and he saves him (but we already knew that one was happening).