I've been thinking more about why he took the bodies of the victims. I came up with the following theory.
This vile man, but very smart and thorough, when planning all this, knew that his truck would be recorded by CCTV cameras at intersections/businesses surrounding the Parkhill area. By leaving the bodies behind, he knew that the police would have been able to determine a time frame during which death occurred. The sooner after death a body is found, the estimation of time of death is more accurate and precise. In this case, with the discovery likely in the morning, we are looking at a very short time frame of possibly less than 2 hours.
With such a short time frame of death the police would look at the CCTV videos showing vehicles in that time frame only and instead of looking at several thousands of vehicles, the police would look at maybe 100-200 vehicles at that late hour. Then, the police would be able to narrow down this number to maybe 3-4 vehicles (considering only vehicles that came in and out multiple times).
Without the bodies at the crime scene, he calculated, the CCTV camera videos would not provide any value to the police.
If he only knew that his vehicle was recorded just a few houses up the road.
In addition to this, by taking their bodies DG avoided the possibility of leaving behind his DNA on the victims' bodies, such as under the victims' fingernails.
More info on this matter at:
http://www.practicalhomicide.com/Research/LOmar2007.htm