I don't think the use of nuclear weapons for surface attacks can be justified in any context because of the global damage they do. I would have preferred if they'd just used chemical weapons to wipe out the same number of people. They are about as effective at wiping out civilian populations, they leave infrastructure intact, and they don't spread radiation everywhere.

Also, there are more atrocious ways to wage war that are less damaging than even chemical weapons. Blinding weapons are extremely effective at pacifying an area and can be fired from planes. Lasers can cause permanent damage to eyes in less time than it takes to blink.

The primary thing holding back such weaponry are the ethical implications of their use. With modern technology, we can do just as much damage with conventional weapons as nuclear, chemical, and biological attacks. Things like drone swarms are a modern conventional weapon that no nation can currently stop. The only way to stop a major world power from taking over the world with drones is by detonating a nuke in the atmosphere above the magnetic north pole and wiping out all electronics in orbit and the northern hemisphere.

In general though, I think that your point of killing all those people to spare more lives is correct. It is the same principle behind Sherman's scorched earth tactics. Do something harrowing to force the enemy to surrender so that more people can be spared from fighting than the act that made the enemy surrender.

You don’t know the horrible aspects of war. I’ve been through two wars and I know. I’ve seen cities and homes in ashes. I’ve seen thousands of men lying on the ground, their dead faces looking up at the skies. I tell you, war is Hell!

- Major General William Tecumseh Sherman