I wonder if that is just time constraints rearing it's ugly head on the plot again. There probably is an interesting story to tell about Dany going from good queen to tyrant, but there isn't time to show it. It could be interesting. She got her start fighting "good' wars, like freeing slaves and fighting an apocalyptic army of the undead. The war in the South is not a good war, but a straight up war of conquest. It would be interesting to see Dany grapple with the realities of such a dirty war and watch as she slowly has to compromise her principles in order to win. But no time for all that, so wham, bam, roast some villagers, she's a baddie now. Oh well.