The Collaborative International Dictionary of English v.0.48:
Instigator \In"sti*ga`tor\, n. [L.: cf. F. instigateur.] One who instigates or incites. --Burke. Syn: inciter, instigant. [1913 Webster]WordNet (r) 3.0 (2006):
instigator n 1: someone who deliberately foments trouble; "she was the instigator of their quarrel" [syn: instigator, provoker, inciter, instigant, firebrand] 2: a person who initiates a course of action [syn: instigator, initiator]