Can you specify which fire bombing campaigns you are talking about? World War II in Europe? Another conflict?

I can have a go at explaining for Japan. In WWII the Japanese were generally regarded as "sub human" and worthy of extermination. For the other axis powers, there were figureheads that were demonised (Hitler), but for Japan it was the entire population. You can see a lot of anti-Japanese propaganda portraying them as insects or giving insects caricatured Japanese features (https://catalog.archives.gov/id/514974) (https://catalog.archives.gov/id/514207), these were often linked with mosquito extermination and called for them to be completely annihilated. This was accepted into soldier's psyche - in Robert Scott Jr.'s memoir he said that every time he killed a “Jap” he felt as though he “had stepped on another black-widow spider or scorpion.” (available here: https://archive.org/stream/godismycopilot005916mbp/godismycopilot005916mbp_djvu.txt)

But, these attitudes were also accepted into the public. Pearl harbour was seen as an unprovoked attack and reports from soldiers talked about how the Japanese were ruthless and would fight to the last man. This gave the impression that it wasn't just the army that was dangerous- but the entire population. Again, lots of anti-Japanese propaganda was pushed through pesticides and so on. Of course, this all ties into "Yellow Peril" at the time, where there was a lot of worry about Asia in general.

As for use of weapons of mass destruction: the US even toyed with using chemical weapons on Japanese, asking "should we gas the Japanese?" (E.P. Russell's War and Nature) which was totally against the Geneva Protocol.