Meh. You could roast everything on the surface with the energy of several large impactors. This would use orders of magnitude less energy than most of the methods described, and still kill every living thing on the surface. (As in everything on the surface is engulfed by molten rock, tidal waves, pressure blasts, and everyone dies in the space of a few hours.)

Lesson: leave the Earth's orbit alone, and just massively disrupt the biosphere.