Most honest answer :)
its my favorite answer
by LachshraeBuild a spaceship, go into orbit around Earth and watch contentedly as everyone kills themselves off. Practically no effort required :D
Edit: I can see a couple things wrong with this on second thought. First of all, anyone looking up into the sky would probably wonder why there was a huge spaceship in orbit, which could be a problem. Also, it would probably get kind of unpleasant being cramped up in a spaceship for years. So I think that I would have to build a self-sufficient base somewhere on the outward-facing side of the moon, to avoid detection.
I would need a way to accurately discern the state of affairs on the planet, so that I could know when it was safe to go down to the surface, and to aid the destruction of humanity a little if it were necessary. I think I would have to have people down on Earth monitoring the state of things and communicating with me via radio waves or something. These people would preferably be highly placed in many governments, and having a lot of power and knowledge of the internal workings of countries. Trusting them would be kind of a problem, though. If one of them decided to go rogue then the whole operation might be brought down. To minimize damage, they would each have to have no knowledge of the others, or who I was and where I was located.
This way, I could sit back and relax in my secret lunar base while other people did the heavy lifting.
I don't know how I would find people to do this for me, though, so that's kind of a problem.
Edit again: I think that I would have to have some kind of implant in their brains, so that I had total control over what they did and what they thought, and have access to all of their memories and knowledge. A computer up on my lunar base would record all of this information, and give the people instructions to do things. The implant would have to self destruct when the person died, to lessen the chance of its being discovered. These people would be like my eyes, ears and hands on Earth.
Then, I would create a poison of some sort that makes people more aggressive and impulsive, with less foresight, and a strong sense of individuality. I would spread this poison to government officials and world leaders using my slaves down on Earth. Then I would spread rumors and create a political scandal about some country making an attack on another country or some such, generally making the global mood less peaceful and more in a state of war. This would get people all hot and bothered, and eventually drive them to launch full-out war on each other. It would be like the Cold War, except that this time it won't be cold.
If this all worked like it was supposed to, the world would soon be destroyed by nuclear missiles, and the population of humanity would be extinct.
Well, but then the world would be covered in radiation, which might be kind of a problem if I were intending to go back to Earth.
I dunno, maybe I need a more subtle method.
holy shit kill yourself