As much fun as it would be to see the World Nobles slaughtered, unfortunately it isn't Oda's writing style to do so. But Teach enslaving them even for a little bit would be sufficient karma, I suppose.
Basically, if he can destroy the WG by conquering Mariejois, then Dragon can start the process of making a better replacement if all the assholes are dead or cowed. Essentially, the bad guy does the dirty work for the heroes.