Two ideas:

-Each turn your faction is in debt, give all generals a "troops want their salary" trait. Lowers morale. When sacking (enslaving or exterminating) a city, the trait goes away.


-This requires that it is possible through scripting to kill generals. If your generals get sick, kill them. This would be more likely if they weren't allowed to rest, and less likely if they are in a city with public health buildings.