On a slightly geographical side note, I don't think Yellowstone would 'kill everything', except within about a 100-150 mile radius of the eruption, but it would certainly completely screw most of North America, and likely a fair bit of Western Europe. It would not be a pleasant time, and the countries south of the equator would make a killing from it (the Mexican border would switch polarities overnight) but humanity would survive, America would survive. It would be a big set back, and there would be thousands, if not millions of deaths (mostly from the ash) but things would continue.
Good description of the likely effects here:
http://io9.com/what-will-really-happ...as-a-508274690