A police official said Yousef Sayhoud was killed on Wednesday in his home in the suburb of Abu Ghraib west of Baghdad.
He said the gunmen also wounded Sayhoud's brother. A health official confirmed the deaths.
They spoke anonymously because they were not authorized to talk to the media.
Sayhoud belonged to the pro-government group known as the Sahwa, which joined U.S. troops in the war against al-Qaida starting in 2005 at the height of the insurgency. Since then, it has been a target for Sunni insurgents still fighting the Shiite-led government, who consider them traitors.