Somewhat related...I did once do some thinking on how I would go about killing all humans if I were a super-advanced alien civilization. I came up with something that wouldn't require any wars or even any damage to the biosphere. It would, of course, require technology far more advanced than ours. There is an emerging field of technology on Earth called "claytronics" which has enormous potential for future applications when mature. The basic idea is to build a large number of microscopic robots (called "claytronic atoms" or just "catoms") and then program them to work together to build any shape required. A claytronic couch could become a claytronic television when needed, for example. It is similar in some ways to the depictions of nanotechnology in science fiction but avoids problems like "how do the molecule-sized machines propel themselves, get energy or know how to read programming?"
Onto the genocide concept. Lets say that the aliens release fine claytonic dust into the atmosphere and water. The catoms are the size of human red blood cells and are programmed to migrate to the brain of any human that inhales them or consumes them. Externally, they might be disguised as tiny dust particles or cellular debris so that they wouldn't raise suspicion if caught on a microscopic slide by accident. They are programmed to remain fairly idle for some set period of time. The aliens could do calculations in advance to see how long it would theoretically take for every human on Earth to be "infected". Once that time period has elapsed, the catoms become active and arrange themselves into artificial blood clots in key regions of the brain. The net effect is that every (or almost every) human being on Earth has a stroke simultaneously. Quick clean kill of billions of people. Humanity would never know anything was wrong until it was too late. The few lucky (or unlucky, depending on your view) ones who managed not to breath or drink any would have no idea what had happened to the rest of the world. There's also no reason the claytronic dust couldn't persist in the environment until even the stragglers were killed (for example, after killing one person they might be released once the body decays. This would allow them to search out new victims).
That would be one way our alien superiors could finish us off if they really wanted us dead. If they have the power of telepathy, shape-shifting, levitation, invisibility, intangibility, mental suggestion, portal creation, etc. then they certainly seemed to be far more advanced that us. Of course, that assumes that these powers are technological in origin and not supernatural or extra-dimensional. So I do suppose it's debatable whether they have the capabilities required to create a claytronic doomsday.
