Will:

is that from Day One, Daenerys has spoken about/threatened/enjoyed the thought of destroying cities for her cause:

Now I’m not sure why people seem to think only villains live in cities and not innocent people but masters and tyrannical nobility don’t only live in the cities she threatens to raze.

Then you have the fact that Daenerys has built up her homecoming in her mind for years, she views herself as a liberator, she views herself as the Breaker of Chains, Breaker of the Wheel, she expects to keep up that momentum when she gets to Westeros but when she gets there, they view her as a foreigner at best and a villain at worst:

Basically, she expected this:

but when she came, they begged the old regime for help, they begged Cersei to ring the bells of surrender, they didn’t clamber to Daenerys and declare her their rightful Queen and liberator, they ran in fear and Tyrion already had to remind her once that the Westerosi aren’t her enemies, when she spoke of them beforehand she spoke of them in terms of not wanting to be Queen of the Ashes, not them as people, and even the first time it’s brought up she hesitates before agreeing

So basically, she has a penchant for burning down cities, the people in Westeros don’t venerate her and instead turned to Cersei, she feels personal loss and feels like she has suffered injustice and so she basically made do on this: