Yeah, but nukes have a bad habit of doing a lot of collateral damage. If the purpose really were to try and get rid of, say, 70% of human population, a nice human-exclusive pandemia would likely be the best choice, assuming that practically slaughtering off most of the people would actually be ethically acceptable - everyone can form their own opinion on that can of worms. Of course, the problem with that is automatically that the poorest people end up dying the most because they don't have the resources to try and avoid the infection.
Obviously the best way would be to decrease population by natural removal. Enforcing one child global policy would perhaps be the single best overall option, aside from the disfigured age structure of the population (young generation ends up as half the size of the previous one), but the problem there would be enforcing such policy on all the people whose well-being is measured in amount of children they can produce to take care of them in their old age.
What comes to question of whether or not illnesses should be treated or not, it's basically a choice between suffering of a specimen and suffering of species. Treat the illness on one person, and you risk making the disease resistant to treatment, plus you increase the population by reducing the natural removal... It's a difficult problem, and I guess there is no right answer - from different ethical points of view, the answer is different.
The most disturbing thing about population growth should be clear to anyone who has taken a look at small rodents' population fluctuations. The population usually grows and grows to some point, until it
collapses. Humans are not that different - we have our ecological locker that can support us to some extent and we may be able to stretch the limits of that support, but eventually if the growth isn't limited by other way... the population collapses due to lack of resources, which will first cause widespread migration periods... hordes of Mongol invaders will be at Europe's back door sooner than later - again. Americas, Australia and GB will probably be better off than Eurasia and Africa, but there'll be internal migration there as well... then the Yellowstone super volcano will erupt and cover the whole continent ona several inches of soot. After that, that pesky Canary island's west side will collapse to Atlantic ocean and cause a mega-super-tsunami which destroys the most of the eastern coast of Americas. Then, the part of California which happens to be on the Pacific plate will sink into the sea. Antarctica will have melted by now, though, so there's not much to sink at this moment anyway, so it'll just be viewed as "good riddance" as it'll finally smother all those bush fires going on in the ultimately dry climate down there. Good point will be the fact that Antarctica can finally be inhabited - the Finnish will have moved there to the remnants of all those Nazi bases hidden in the bottom of the ice aside with the ancient Predator-Alien temple, where they'll invoke the slumbering beasts and then fight them to standstill like always. At this point, though, there are no more national states, though, since the states will lose their legitimacy when they lose their law enforcement power and people will go back to living in small, xenophobic communities that fight for territory. And don't even ask what happens to all those ***** nukes lying around the world in missile silos, they'll probably be used at some new years' celebrations, not that anyone will have much of a reason to celebrate at this point, nor they have much clue what year it actually is, since all the different calendars will have mixed up by now and probably people start using years from the coronation of the local king. Not that it matters for most people anyway since they can't read, write or do much else than try and stay alive as best as they can until they die at the age of 25 at the hands of mutated killer Kiwi.
After a lot of miserable existence, the humanity will finally rebound and stabilize at estimated <1 billion, and the history will tell if the idiots still fall for the idea of national states and other stuff. Probably.
The good thing is that the humanity will lose it's power to goof up with the ecosystem for decades or even centuries, and wolves and bears and other beasts of the forest will have a lot better time in the lush forests of Antarctica.
...ummmkay, it seems my future prediction got a little out of hand. Feel free to ignore that little bit flow of consciousness from my part...
EDIT: Oh, and forgot to say that there'll be a whole horde of new and old religious zealots that will demand the humble servitude for the galactic overlord Xenu, Flying Spaghetti Monster and probably there'll even be a religion based on Silmarils, Valar and other actually good stuff, for who the hell will be able to tell parts of Silmarillion from the Bible if they have no previous knowledge of either...?
