The soldiers of the Japanese army were absolutely insane thanks to military brainwashing, and they would have fought us and any Japanese citizens that tried to oppose them tooth and nail to the death. Whether it was morally acceptable to nuke them is up for debate, but there is no doubt it brought the war to an end sooner than it would have otherwise ended and saved American and Japanese lives.
The cities bombed weren't arbitrary targets, either--they were militarily significant cities.