I'm on phone so no quotes, sorry.
No one left to fear her? It's just a one city after all. Doesn't Westeros have like many kingdoms? Now there's definitely reason to fear her. For a moment her dragons looked vulnerable.
And yes I still say she sees enemies everywhere. Obviously she won't tell Jon, the rightful heir, that. Her options were pretty much to accept the surrender which would likely lead Jon taking the throne or choose the fear and become a tyrant. Of course I don't think she had a clear plan, she hasn't had one for a while. She's been portrayed to be driven by the hunger for power and anger for a good while.
She executed Varys without emotion. Tyrion plead her to stop the attack if she heard the bells but you could tell she wouldn't accept it. And Robert Baratheon said the world would burn if Dany came to Westeros :P There was no need for her to go full HEERE'S JOHNNY at any point, we've seen her slowly descenting into madness.
There's plenty of other poorly written plotlines or retconning in latest seasons and yeah everything feels a bit rushed. This one was fine though. Predictable, but fine.
edit. Oh and that Sansa example is a bad one. Completely different characters. There's a reason why so many viewers including myself have been thinking Dany is being corrupted by power and going mad for many seasons. Nobody expects anything like this from Sansa.
Looking back I don't remember her showing any emotion when executing people going right back to Season 1 when her brother died in a most brutal fashion.
She has the ability to switch off all emotion and de-humanize someone if they don't follow her.