More specifically, how this season (and the last season, to an extent) has completely rushed to this conclusion without satisfactorily portraying the decline of Daenerys' mental state.
Even if you wanted to make the argument that Dany's sanity has been systematically declining (which is something we've been told more than shown in recent years), there's nothing in her past or recent behaviour to provide a logical reason for why she would suddenly turn on thousands of innocent people after the Lannisters had surrendered. It might be more understandable if she had flown straight to the Red Keep to attack Cersei directly, but her zig-zagging her way through King's Landing is frankly baffling.
Daenerys has always fought to protect innocent lives. For all her bluster and her violence, she has at least always believed she was doing the right thing. There was no evidence this was the case in "The Bells" – rather, she was portrayed as acting out of fury, apparently seeking vengeance. But her target made no sense, unlike those that have come before.