Team Killer is a Freak created by Lemurfot, who based it on many other real psycopath movies like Freddy Krueger, Jason Voorhees, Leatherface and Michael Myers. He is also has a nickname "Rave Killer" due his killing of others at dance clubs.
He used to be a normal Scout with a family, one day a group of RED team members attacked him and his daughter who was killed. He sought revenge by attacking anyone wearing RED, hoping to get revenge on the thugs. It isn't clear why the RED members attacked him even though Team Killer was not a blue Scout.