Same word as calumniator:

adversary, agent, antagonist, archenemy, asperser, assailant, assassin, attacker, backbiter, bad person, bandit, betrayer, calumniator, competitor, contender, criminal, defamer, defiler, detractor, disputant, emulator, falsifier, fifth column, foe, guerrilla, informer, inquisitor, invader, murderer, opponent, opposition, other side, prosecutor, rebel, revolutionary, rival, saboteur, seditionist, slanderer, spy, terrorist, traducer, traitor, vilifier, villain