Be prepared. Evil/unscrupolous characters can do plot U-turns so quick they'll leave you spinning.
For example, a character I was GMing was kidnapped by an insane ulitharid, who brutally tortured him, and vivisected several of his companions in front of his eyes.
After escaping, the BBEG would occasisonally appear and say 'mwah-ha-ha, come with me an I will unlock awesome power from within you for both of us mwah-ha-ha'
After only the second appearence, much to my suprise, the PC said 'sure !" and went with the BBEG. Needless to say, his former party members that had survived the BBEG's tender loving torture were not entirely pleased with his choice.
Motivation can be either tricky or easy. The character mentioned above is motivated by lust for power, which he uses to further his primary goal of killing everything he defines as 'monsters'.
Come to think of it, most of the evil/unscrup cahracter' I've played have been motivated by power, too. Moral of the story : make sure the players work out good goals and motivations for themselves
The old adage 'make it personal' is a good way to go. Kidnap
their families, rain terror on
their homelands (good for LE patriots).
If they're powerful enough, make the good guys come to the pleading for help.
"Please, Sir Viledoom the Unspeakable, you are the only one who can stop the Demiliche Nastypants from collapsing the prime material into the nearest black hole !"
"Hm. What's in it for me?"
"...you get to live?"
"Ah. Okay, but I ant the princess, eight metrica buttloads of treasure, and....
your head on a platter"
Good stuff
