1) An assailant or invader 2) Assailant 3) Assaulter 4) Attacker 5) Attacking force 6) Bellicose one 7) Offender 8) Someone who attacks 9) Wrongdoer Found on https://www.crosswordclues.com/clue/aggressor
Ag·gres'sor noun [ Latin : confer French 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. » Found on http://www.encyclo.co.uk/webster/A/43