These murders might be like the DC sniper's case. He shot his girlfriend and then went on a shooting spree to make it appear she was randomly killed like his other victims. (Someone who was owed money or child support may have been the target, but the rest of the family was killed to confuse the police.)
I'm still having a difficult time believing a 19 yo guy was able to stump the FBI, Homeland Security, et al for over a year by pulling off the execution of 8 people in 4 locations.