Lamont Lewis is testifying Monday at Savage's federal death-penalty trial.
Lewis says he carried out the murders because - quote - "that's what I did for the team."
Savage is charged with killing 11 people during a violent reign in the North Philadelphia drug world. He is already serving 30 years for a drug kingpin conviction.
Lewis admits taking part in the 2004 firebombing that killed two women and four children related to suspected informant Eugene Coleman. The arson is among the worst retaliation killings in city history.
Lewis says he killed the Coleman family on orders from Savage and his sister, Kidada Savage, who is also on trial.