Source : Webster's Revised Unabridged Dictionary (1913)

Aggressor \Ag*gres"sor\, n. [L.: cf. F. agresseur.]
   The person who first attacks or makes an aggression; he who
   begins hostility or a quarrel; an assailant.

         The insolence of the aggressor is usually proportioned
         to the tameness of the sufferer.         --Ames.

Source : WordNet®

aggressor
     n 1: someone who attacks [syn: {attacker}, {assailant}, {assaulter}]
     2: a confident assertive person who acts as instigator