Sometimes, I just want to play bad just to be bad.
Like playing the Joker in any Batman game that lets you.
If the character is psychotic, then destroying the world (and yourself in the process) isn't unrealistic.
The psychology of it is that even though the world is gone, YOU did it, proving that YOU are the most relevant / powerful person ever.
Of course, I'd make the game have multiple endings based on events.
Really, it could just even be a scaled-up version of Counter-Strike.