Dany committed a War Crime.
There is no justification for it. If there was a justification, then it would not be a war crime. This is what GRRM wanted, Dany commits a war crime. It was one person who made a decision to use a weapon of mass destruction on innocent people.
It is sad. I did not want this to happen, but it was told to us from the beginning, and even the last episode, that this was a strong possibility.
GRRM is not writing a story about good vs. evil. He is writing about War, Power and Peace.