The Collaborative International Dictionary of English v.0.48:

Betrayer \Be*tray"er\, n.
   One who, or that which, betrays.
   [1913 Webster]

WordNet (r) 3.0 (2006):

betrayer
    n 1: one who reveals confidential information in return for
         money [syn: informer, betrayer, rat, squealer,
         blabber]
    2: a person who says one thing and does another [syn: double-
       crosser, double-dealer, two-timer, betrayer,
       traitor]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "betrayer":
   Benedict Arnold, Brutus, Judas, Judas Iscariot, Quisling,
   archtraitor, blab, blabber, blabberer, blabbermouth, cockatrice,
   conniver, conspirator, conspirer, convict, criminal, crook,
   debaucher, deceiver, defiler, delator, desperado,
   desperate criminal, despoiler, double agent, double-crosser,
   double-dealer, felon, fink, fugitive, gallows bird, gangster,
   gaolbird, informer, intrigant, intriguer, jailbird, lawbreaker,
   machinator, mobster, narc, nark, outlaw, peacher, plotter,
   public enemy, quisling, racketeer, raper, rapist, rat, ravager,
   ravisher, schemer, scofflaw, seducer, serpent, snake, snitch,
   snitcher, spy, squealer, stool pigeon, stoolie, swindler,
   talebearer, tattler, tattletale, telltale, thief, thug, timeserver,
   traitor, treasonist, trimmer, turncoat, two-timer, violator,
   whistle-blower