troublemaker

from WordNet (r) 3.0 (2006)

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

from 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