I'm voting for none other than yours truly being the cause of the earth's destruction. Here's how it'll go:

1: I start my own cult
2: Make millions off of gullible idiots that join my cult
3: Use the money to invest heavilly in a space program
4: I take over the moon
5: Build a "+5 super-atomic mega laz0r of smiting" on the moon
6: lay siege on the earth, and in the process destroy it
7: eat all my cultist minions because I suddenly feel a little hungry
8: I'm the only one left!