noun
betrayer, traducer, deserter, renegade, Judas, quisling, Benedict Arnold, Brutus, fifth columnist, informant, informer, intriguer, spy, double agent, counterspy, collaborator, collaborationist, hypocrite, imposter, plotter, conspirator, turncoat, sneak, recreant, backslider, double-crosser*, wolf in sheep's clothing*, fink*, rat*, stool pigeon*, two-timer*, copperhead*, stoolie*; see also rebel 1.
Antonyms supporter*, follower, partisan.