Possibly one of the simpler theories regarding the end of everything is The Big Crunch. This is basically "what goes up must come down" on a cosmic scale. The theory assumes that the density of the universe and the gravity it generates will eventually be enough to slow the expansion of the universe and put it into reverse. If this were to happen, then everything that is currently flying apart at unimaginable speeds would about turn and start hurtling towards each other instead, logically leading us back to a point where all matter in the universe exists at one infinitely small and dense point - a singularity. There are a couple of problems with this theory, mainly that it makes too many assumptions. It assumes that the universe is finite, closed and doesn't take into account any of our more recent enquiries into the quantum nature of the universe (specifically, quantum gravity). This doesn't make it impossible, but if you were looking to destroy the universe, this option might make you look like a bit of an amateur.
