The problem with the evil population-controlling chemicals idea is that it would be about the worst possible method of spreading them. There would be no way to tell where they’d eventually come down; it could literally be on the other side of the world. There’s no way to regulate the dosage. There’s no way to keep the chemicals from getting caught up in rain clouds and diluted into uselessness, or frozen, or fried by the sun, or whatever else. They would also be really easy for any private lab to discover. It would be pretty much useless.
It would be potentially valid as weather modification, but that’s not a very useful thing itself overall. The USSR made some huge trials with that sort of thing, and it ended up being minimal gain for an immense amount of work. Besides which, the side effects (such as everyplace downwind having inky rain that stains everything black) would make it incredibly obvious.
