Source : Webster's Revised Unabridged Dictionary (1913)

Instigator \In"sti*ga`tor\, n. [L.: cf. F. instigateur.]
   One who instigates or incites. --Burke.

Source : WordNet®

instigator
     n 1: someone who deliberately foments trouble; "she was the
          instigator of their quarrel" [syn: {provoker}, {inciter},
           {instigant}, {firebrand}]
     2: a person who initiates a course of action [syn: {initiator}]