I third Monty. The best way for you to understand why your antagonist is doing what he's doing is to get inside his head. Of course, for you to understand him, his thoughts/feelings/motivations need to be somewhat humanistic. There doesn't necessarily have to be something in his past that explains his need to destroy (whereas, we'd need that for a human), but his reasons could have foundations in human emotion. Maybe he's just greedy for power, and destruction is what he does, so the more he's able to successfully do it, the more powerful he feels.

Cheers,
Michelle