Interesting article, on Slate of all places, looking at how Daeny may have not gone mad, but been making a tactical decision.
Slate: The Strategic Case for Burning King’s Landing.
A Military Strategist on Why Daenerys’ Scorched-Earth Policy May Not Be So Mad After All
It seemed pretty clear to me that she was pissed that Jon wouldn't lay the pipe and so she took it out on the entire population.