(n.) One who, or that which, betrays.

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