First define what evil is, then discuss with players what they want to do as evil players, what are their goals?
Ideas for games...
Kidnapping - standard fare
Stop the good guys - hey, they want that item? So do we!
Pick up the taxes - someone has to.
Run the farmers off their land - road coming in, or a strip mine.
Lower properity values - as above but in a city, mug a few people, set fire to a few places.
Kill the prince - oh, he is a goody goody and will be an issue but his brother is easy to work with.
Need slaves - nuff said.
Effect value of stocks and food - burn down farms, silos, hi-jack trade carts, sink a few ships.