Climate change is accelerating at a terrifying pace, globalization is pushing more and more of the world to consume at an American scale, and world populations are growing at rates unimaginable just 200 years ago. Beyond CO2 being dumped into the atmosphere at alarming rates, massive amounts of pollutants, toxins, and substances that take a very long time to break down naturally are flooding the ecosystem. And even if those problems are magically taken care of, massive habitat destruction is placing immense stress on the ecological balance between flora and fauna, resulting in a mass extinction event only experienced on this planet for a handful of times in its multi-billion year existence.
From this point, a committed actor might find the idea of killing off most of humanity to be an attractive option. They might be a very wealthy individual or group seeking to enjoy a paradise planet free of the lower classes, a radical inspired by religion or environmentalism, or what have you. Point is, they've come to the conclusion that saving the Earth while saving humanity might require unprecedented genocide.
As far as conditions go, they aren't looking to kill off everybody. Just...the "rest." They want a functioning society like we have today (so no eco-hippies looking to live like Adam & Eve) with all of the technology currently in use. They just don't want so many people on the planet that their existence and consumerism will poison the planet for everybody. The best-case scenario is one where most of the people in the world simply disappear and the survivors - for the most part - are ok with this.
BUT THEN as this person or group plays out the set of conditions that would end the life of 90%+ of people (while ensuring no significant harm is done to the greater ecosystem), they find that this plan - even if correctly carried out - will NOT work out for them.
Do they reason out that too few survivors would be unable to repopulate humanity without nasty inbreeding side effects? Would they be unable to maintain the many complex parts of modern life (electricity, telecommunications, civil engineering, etc) and ultimately lose thousands of years of knowledge and regress into a new Dark Ages? Would they predict that infighting among the survivors would be impossible and lead to either of the situations above?
What prediction or realization would cause a very determined group to completely abandon otherwise well-planned initiatives to kill off most of the people on Earth? Given how determined these people are, it would take an absolutely devastating endgame for them to completely abandon their plans. So while it might get boring watching re-runs of the same television shows over and over, it won't be a deal breaker. It has to be something so terrible that it would force the planners - who are really into the mass genocide idea - to collectively quit the plan for good.
Bonus points for viable alternatives to mass genocide that plausibly protect the planet and the modern middle-class way of life.