It seems the killing ring seems to be getting bigger. Could there be an ulterior motive for that? And note, there is nothing going on right in D.C. itself. Maybe these killings are just the diversion.
This huge manhunt has tied up just about every available law enforement agent in the area, and they are no doubt getting tired and strung out. What if there were a few more successful hits and then ... a final shooting (or even two or three nearly simultaneously) - still way out in the suburbs and away from D.C., pulling the cops out even further and tying up the roads and promoting chaos.
Then, the primary attack takes place. Except this time it is against a major target in downtown D.C. - like the Capitol, White House or Supreme Court? The reaction of backup security would not be so good, would it, with most of the players and their backup out in the suburbs, an hour or two away. This would give time for the main assault by a large team of killers.
This large assault concept has been pulled before by terrorist groups in Latin American countries, IIRC. Remember, the last plan on 9/11 seemed to have some earmarks a political decapitation was planned.
Just speculating on another possible motive for 'what' and 'why'.
Killers do multiple shootings in different areas, the conditioned response to shut everything down with everyone out in the open happens and bang- WMD goes off.