The lack of natural resources is the root cause of conflict between the humans. So in order to make the planet a better place to live, we need to do two things: 1. Reduce the human population so that the per capita availability of natural resources increase 2. Make better distribution of the natural resources.
Exactly the reason why I would want to make a pill that makes a person infertile. I would just distribute it with free food to all poor people so atleast poor people who can't afford to raise kids, won't have any.