try to look at it from the other perspective

too many people making too much pollution
greenhouse gasses adding to the atmosphere at an alarming rate
a mass extinction event underway

so whats the most expedient solution?
spray stuff in the upper atmosphere to deflect more radiation from the sun. It also happens to bio-accumulate and cause the premature death of the population, especially if you add combinations of chemicals or viruses designed for this purpose. So the net effect is to reduce the effect of the greenhouse gas already in the atmosphere while reducing the primary cause of the greenhouse gas emmisions. It's a win win from the scientifically anti-human perspective.

Also the radiation from the various nuke tests and nuclear plant disasters and from the depleted uranium ammunition used in war, it all has an effect. It causes birth defects and cancers and all manner of problems for the people of the world and they use the ciggarete companies as a scapegoat putting the blame on nicotine for cancer instead of considering that the military may have been to blame for the rise of cancer and how radioactive our world is.

also the medical/pharmaceutical industry allows them to make a large profit from the treatment of the cancer they caused. This money goes to researching new medical treatments which will result in life extension for the super-wealthy, possibly even immortality. Now consider that you can't have 7 billion immortals running around making babies for the forseeable future.

Also consider the advance of computers, robotics, artificial intelligence. By 2020 the desktop PC would be smarter and more capable than your average human produced by this idiocracy system we live under. Clearly the few who already have wealth and power just want to get rid of about 90% of the excess human population so they can go about their goals of being immortal, restoring the environment to pristine condition and generally living the good life like they envisioned in Star Trek. Robots will do all the labor and manufacturing and they can focus on study and creativity and exploration of the universe.