(n.) One who makes mischief; one who excites or instigates quarrels or enmity.

agitator agitprop capper catalyst come-on man decoy demagogue exciter firebrand fomenter incendiary inciter inflamer instigator mischief-maker plant provocateur provoker rabble-rouser ringleader rouser seditionary seditionist shill stool pigeon stoolie troublemaker urger