WordNet (r) 3.0 (2006):

troublemaker
    n 1: someone who deliberately stirs up trouble [syn:
         troublemaker, trouble maker, troubler, mischief-
         maker, bad hat]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "troublemaker":
   agent provocateur, agitator, agitprop, catalyst, demagogue,
   exciter, firebrand, fomenter, gadfly, incendiary, inciter,
   inflamer, instigator, malcontent, meddler, mischief-maker,
   provocateur, provoker, rabble-rouser, ringleader, rouser,
   scandalmonger, seditionary, seditionist, stormy petrel, urger