The 4th Edition Champions Adventure Day Of The Destroyer had a particularly grandiose--and horrifying scheme in which Doctor Destroyer announced that he would kill nine out of every ten people, stating that with the population much reduced, there would be less damage to the ecosystem and more resources available for the survivors. Not unlike the first Kingsmen movie, in which Samuel Jackson's tech genius character wanted to use a mass mind control system to make everyone in the world beat each other to death, thus saving the planet from humanity, and the James Bond movie Moonraker, in which Hugo Drax would wipe out all humanity with some sort of biological agent, enabling him to repopulate Earth with his own race of genetically superior beings.
I've always figured that a mass human extinction scheme would bring much more harm to the ecosystem than help. First, you have all those billions of corpses and no way to completely dispose of them--and as they decay they become incubators for disease organisms, which would spread unchecked through the surviving humans. Second, there would be an explosion in the vermin population as all the rats, mice, insects and scavenger beasts would suddenly have an overabundant food supply--and as they fed, they would breed all out of proportion of their predators' abilities to keep them in check. And once the supply of dead bodies is consumed, those vermin would then turn to the living to survive--and that includes surviving humans.
Hope that helps