Nuclear weapons are considered immoral because they are capable of killing millions of people with one shot; they kill noncombatants along with combatants; they utterly and completely destroy the target and everything around it, usually for many miles in circumference, depending on the size of the weapon; they pollute the area where they are employed for thousands of years; and the residual fallout can drift over many countries, killing innocent people who had nothing to do with the belligerants who were the target of the weapon. If enough of them were to be used, the resulting nuclear winter could mean the literal extinction of all higher life as we know it on the planet.
When I was in the Air Force, the NBC Warfare (Nuclear, Biological, and Chemical) classes we had to take went into nuke effects. Somebody asked the officer teaching the class, "Would life survive an all-out nuclear war?"
He kind of smiled, and said, "Yes, but it would have to meet certain criteria. Any biological organism, in order to survive an all-out nuclear war, would have to possess the following characteristics: it would have to be extremely radiation resistant to survive the fallout; it would have to be capable of hibernating for extended periods of time, to survive the nuclear winter; it would have to be capable of reproducing quickly and prolifically, in order that it didn't die out; and it would have to be not really too picky about what it decided to eat."
He paused, and then said, "Are there biological organisms right now which meet all these criteria? Of course. Cockroaches; flies; earwigs; and other such organisms that we consider pests would thrive in such an environment. A human being can only take about 150 rems and survive, if he's healthy and strong; anything higher and he's dead. A cockroach can absorb upwards of 5,000 rems and never show any ill effect. Roaches can also burrow into the ground and remain dormant for years, if need be. They also breed like mad, and a roach will eat anything. Upon emerging at the end of the nuclear winter, the roach will find billions of rotting human and animal corpses to feed on, lay eggs in, and the residual radiation won't bother it one bit. Same for flies, same for many kinds of beetles. In short, the bugs will inherit the earth."
We just sat there and sort of blinked a few times. The officer shrugged and went on with his lecture.
I don't know about you, but any situation where human beings are replaced as masters of the planet by cockroaches is immoral, as far as I'm concerned.