WordNet (r) 3.0 (2006):

bad guy
    n 1: any person who is not on your side [ant: good guy]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "bad guy":
   Roscius, actor, actress, antagonist, barnstormer, character,
   character actor, character man, character woman, child actor,
   diseur, diseuse, dramatizer, feeder, foil, heavy, histrio,
   histrion, ingenue, juvenile, matinee idol, mime, mimer, mimic,
   monologist, mummer, pantomime, pantomimist, playactor, player,
   protean actor, reciter, soubrette, stage performer, stage player,
   stooge, straight man, stroller, strolling player, theatrical,
   thespian, trouper, utility man, villain