2E:
I had a party defeat an evil overlord type at the end of a long adventure that decided to make the most of it. I let them pull together an army from the newly freed lands they now oversaw, and they soon began a series of complicated raids as well as employing spies on the king they were sworn to obey. Between sessions I thought of having the tables turned on them by a young new adventuring party that would foil them at every move, just as they had done. Soon enough their schemes got them into hot water before they could get help from some of the king's long time enemies due to this adventuring band, and they became the bad guys. The last dual between the parties was on a battlefield with the opposing armies, and involved the king (who survived although he lost the fight) and the party was banished from the kingdom.
I gotta say that the players were quite wily and thought out every move they made to thwart the adventuring party they began to detest, just like bad guys in the movies, and they cackled when they killed all of them in the final battle. The PC mage even sacrificed himself to get the last member of the upstart group, a thief that was trying to escape in the midst of battle. It was a great adventure that even now they talk about with much appreciation for the bad guys they now seek to replace.