Carlin's podcast on this subject, summarizing other well-researched histories, gives a much better idea of why firebombing and the atomic bomb were considered within bounds. The Japanese government was willing to throw away an appalling number of military and civilian lives in order to delay inevitable defeat, or at the very least attempt to bargain for better terms on which to end the war. (Carlin accurately describes this as "a form of political murder.") In places like Burma, New Guinea, and Peleliu, Japanese soldiers were thrown into utterly hopeless actions in order to inflict as many Allied casualties as possible. Beyond this, Japanese soldiers generally refused to surrender, and often had to be killed to the last man.

Firebombing and the atomic bomb, while obviously subjects of internal bureaucratic and ideological jockeying, were considered viable primarily because they might shorten a long and very brutal war. Whether they did so or not is debatable, but we'd be on very shaky ground to debate the morality of the attempt from the comfort of posterity.

I don't particularly care for Gladwell's glib nonsense, but I also find fault with the sort of anti-Gladwell Brooklyn-podcaster history (e.g. https://thebaffler.com/latest/narrative-napalm-kulwin) that engages in righteous hand-wringing over supposed American atrocities, without once mentioning any of the context above, nor Japan's atrocities in China in the 1930s leading up to the conflict, nor the very obvious fact that America didn't start the war.