Reasons to destroy the world:
1. You hate existence and want to end it completely.
2. You were against the original creation of the world, and would like to see it unmade.
3. You prefer the undead.
4. You want to own the world, but first you need to implement a few changes (e.g. putting out the sun, lowering the temperature, covering the planet in water, or setting the planet on fire).
5. You want to return to your home dimension. Unfortunately, your plan involves the destruction of this one.
6. You are predestined to play an important role in the end times, and you are overeager to see them start.
7. You literally eat planets.
8. You hate all other forms of life, and you decide to exterminate them all.
9. You want to prove that you are smarter than everyone else, and your megalomania has gone a little overboard.
10. The world is an important spell component for the immortality spell you have devised.
11. You are under the control of alien beings who wish to use this dimension for their own purposes.
12. You think that destroying the world will elevate you to godhood.
13. If you were to kill everyone, the world would finally be at peace.
14. You are extremely powerful, and sometimes you destroy worlds accidentally.
15. You were programmed to kill.
16. You want to hold the entire world for ransom.
17. You believe that this world is a prison, and its destruction will mean freedom for everyone who lives here.
18. You really enjoyed your trip to the far realm, and you're pretty sure everyone else will enjoy it, too.
19. You have been wronged, and this is your vengeance.
20. You are simply evil, and destroying the world seems like an evil thing to do.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.