There are plenty of possible reasons for wanting destruction. Maybe he came from a small, destroyed land, constantly getting screwed by it's larger, aggressive neighbors, and he feels for his land to thrive the others must be knocked down a few pegs. Perhaps the motive could be greed, although on that large of a scale the idea doesn't really track well. Tearing everybody down so your life is better by comparison is really petty and stupid, and if he has the resources to wage this war he probably is living pretty well in the first place. He could be preparing for a terrible threat, kind of like the antagonist in Fable 3 (not my favorite game, but a decent point made.) He could be doing horrible things now in order to protect the world from a greater threat later.
My personal favorite is the first one. He could feel the need for success because he was raised in a defeated land, where failure was imminent. If he was raised a failure, the desire to prove himself, and his country, would be overwhelming.