WordNet (r) 3.0 (2006):

defector
    n 1: a person who abandons their duty (as on a military post)
         [syn: deserter, defector]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "defector":
   apostate, backslider, bolter, collaborationist, collaborator,
   convert, deserter, fifth columnist, mugwump, proselyte, quisling,
   rat, recidivist, recreant, renegade, renegado, renegate,
   reversionist, runagate, schismatic, seceder, secessionist,
   separatist, strikebreaker, tergiversant, tergiversator, traitor,
   turnabout, turncoat, turntail