There are at least 4 bands named Aggressor.
#1: Aggressor is a Canadian thrash metal band formed in Ottawa, Ontario in 2006. Today it consists of Brain Stephenson (Annihilator, Sanctity), Adam Pell, Max Gingras and Chris Stephenson.
#2: Aggressor was formed in Estonia back in 1989. In the beginning of 1990 they "released" their first rehearsal demo "Indestructible" - it had strong influences of Kreator. Their first official gig was on April 17th, 1990. Read more on Last.fm