The planet itself will more likely go in one of three ways- The expansion or destruction of our sun, a meteor large enough to destroy the planet, or a black hole.

Society, on the other hand, will most surely go out with a couple nukes or an epidemic plague. Both a plague and a nuclear attack, are the most easy to pop up right now. Nuclear proliferation is speeding up, and our entire system of healthcare is stiffing up. You get something like the bubonic plague from the dark ages, and no one's gonna avoid it. The ENTIRE PLANET will suffer. As for nukes, only one or two super nukes is required to cause a nuclear winter, which would wipe out life on the planet.