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]