defector

from WordNet (r) 3.0 (2006)

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

from 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