Flowers was killed as he was preparing to testify against Savage in another murder case.
Defense lawyers Thomas Egan, William Bowe, and Michael Wiseman called Northington's mother and medical experts to bolster their claim that he suffered from limited mental capacity, a situation compounded by his upbringing in a poor, drug- and crime-ridden swath of the city.
Savage, 38, is awaiting execution for 12 murders, including the deaths of Parker and Flowers. He also ordered the 2004 firebombing of a North Philadelphia home that killed two women and four children related to another witness.
Two other defendants, including his sister Kidada, face life in prison when they are sentenced this year by U.S. District Judge R. Barclay Surrick.
A senior Justice Department official said officials hope the verdicts bring some measure of justice to the victims in the case.
"For more than a decade, Kaboni Savage and members of his organization used murder and violence to intimidate and retaliate against anyone who threatened their drug trade," said acting Assistant Attorney General Mythili Raman, "and along the way mercilessly killed a cooperating witness's family members, including innocent children."
Contact John P. Martin at 215-925-2649, at firstname.lastname@example.org or @JPMartinInky on Twitter.