The Collaborative International Dictionary of English v.0.48:

Turncoat \Turn"coat`\ (t[^u]rn"k[=o]t`), n.
   One who forsakes his party or his principles; a renegade; an
   apostate; a defector to the enemy.
   [1913 Webster]

         He is a turncoat, he was not true to his profession.
                                                  --Bunyan.
   [1913 Webster]

WordNet (r) 3.0 (2006):

turncoat
    n 1: a disloyal person who betrays or deserts his cause or
         religion or political party or friend etc. [syn:
         deserter, apostate, renegade, turncoat, recreant,
         ratter]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "turncoat":
   Benedict Arnold, Brutus, Judas, Judas Iscariot, Judas-like,
   Quisling, Trojan-horse, Vicar of Bray, apostate, archtraitor,
   backslider, betrayer, betraying, bolter, cockatrice,
   collaborationist, collaborator, conniver, conspirator, conspirer,
   convert, defector, deserter, double agent, double-crosser,
   double-crossing, double-dealer, fifth columnist, fifth-column,
   informer, intrigant, intriguer, machinator, mugwump, plotter,
   proselyte, quisling, quislingistic, rat, recidivist, recreant,
   renegade, renegado, renegate, reversionist, runagate, schemer,
   schismatic, seceder, secessionist, separatist, serpent, snake, spy,
   straggler, strikebreaker, tergiversant, tergiversator, timeserver,
   traitor, traitorous, treasonable, treasonist, treasonous, trimmer,
   turnabout, turntail, two-timing