1. Make a killer virus
2. Use the ensuing pandemic to scare people
3. Shoot them up with a vaccine
4. Ensure that the vaccine renders most of the people who take it much less fertile than they otherwise would be while reducing their lifespan
Result:
Depopulation on a global scale.
Rainbow Six by Tom Clancy. The killer flu was to have been released, and a vaccine was supposed to then appear on the scene - which was a worse version of the killer flu. The purpose was extreme mass depopulation.
Remember, these envirowackos believe that humanity is a virus infecting the Earth Mother Goddess Gaia.