As vindictive and vengeance seeking Daenerys was over 8 seasons, she never sought to spill blood for the sake of spilling blood. Outside of the Tarlys, you could come up with a reasonable justification for just about every act of violence she perpetrated, because it was almost entirely directed at slavers, traitors and backstabbers.
This is a woman that kept her dragons chained in a dungeon for a year because one of them burnt a child alive. There needed to be a seasons worth of development to show the descent from that woman to the fire breathing lunatic we saw last night.
I think if she'd flown to the Red Keep and incinerated it, that wouldve been in keeping with her character, but the episode had her circling the city and systematically reducing it to rubble and ash for seemingly no reason other than to shock us, solidify her heel turn and mentally prepare us for whatts looking to be an ugly and violent demise for her next episode.
The entire episode was incredibly shot though, arguably the best directed episode of action tv ever. It's a shame the actors, directors and talent behind the scenes was let down by a rushed script, WHICH THE WRITERS DELIBERATELY CHOSE, since they turned down an HBO offer of another 10 episode season.