you're reading a lot into the scene here, are you aware of that? we don't really know why she just decides to burn innocent people - is it because she's having a psychotic break somehow triggered by bells, is it because she was so filled with rage that she just wanted to kill people no matter what, is she trying to instill fear in the populace regardless of their surrender, is it revenge for the people of westeros not loving her enough again regardless of their surrender?

we just don't know, and the fact that we don't know something this important in the penultimate episode of the series is as good of evidence as you will ever see of shitty writing.