I had always thought that we would kill ourselves with war and bombs, but three out fo the seven major reasons (#2 Sun Storms, #5 Super Volcano, and #7 Slip-Slop-Slip Bang) indicate it is nature that will kill the human race. The only scary one is the Slip-Slop-Slip Bang which is the most possible to happen, and will not only kill all of humankind, but all life.
Remember though, that these are just theories, and will not necessarily happen, so don't start going crazy and doing whatever you want in 2012, the time wont necessarily come.
The again, which theorie will happen first, if they were all going to happen? Will they happen?
I would like to see if any of these theories actually kill us in four years time.