The Mad Duke began his conquests 15 years ago, and erects many statues of himself in the towns he takes over. Thereafter the townspeople grow increasingly paranoid, reading portents in everything and distrusting of their neighbors.

He's also a very proud horseman.

The Mad Duke thinks the gods have a soft spot for the dwarves - a soft spot for the wise people. He thinks the gods love dwarves so much, they will stop him from taking over the world. As a result, he kills dwarves on sight.