How to completely wipe out mankind

I just came from Hacker News, where there is a short discussion on how many nukes would it take to wipe out humanity. This is an expansion of my post there.

The original article reaches the conclusion we don't have nearly enough nukes. I disagree. It's a matter of using them efficiently.

I guess a couple hundred average nukes would be more than enough to nudge a passing rock (there are many readily available) into a collision course to Earth. Better yet would be to use the fissiles to power a mighty big NTR attached to a comet and use its own water as propellant. This would be a lot more discrete than a big blast, may provide for a quicker intersection and also add a nice oomph to the comet for when it hits the ground. I would also suggest hitting the Atlantic or Yellowstone as nice strategies for maximizing destruction. Hitting Europe could yield the maximum number of immediate deaths, but the other two may provide a larger overall devastation. I bet a comet hitting a supervolcano is a very memorable event.

And nothing, apart from the supervillain's budget would prevent the use of more than just one comet. A string of fragments, like SL9, could rain death very evenly across all inhabited places of the Earth.

In the end, it all depends on how long you are willing to wait until the last human is dead. If you require them to be all vaporized by the end of the afternoon, then, perhaps, we don't have nearly enough nukes. If you are a patient villain, you could use your resources far more efficiently to first render useless a huge part of our infrastructure (with EMPs from high-altitude detonations), then ruining food supplies (even small nukes could start fires) and only then using the remaining firepower to wipe out whoever is left after they have had a couple years to restart civilization and concentrate a bit.

It seems very doable. In fact, we may even be able to wipe out humanity without using any nukes. I regard some politicians as much, much more destructive than simple nukes.