If he indeed was the driver that caused the death of Matthew Kemper Hartley (and his sister), and felt extremely guilty ("traumatized after causing what he described as a horrific accident after falling asleep behind the wheel"--again if the two accidents, one involving MKH, one involving DG, were the same accident), then there is the possibility (might make some sense) that he felt so guilty that he thought by assuming his victim's identity he made him (MKH) continue to "live on"; of course it also helped him hide as the culprit of the accident, like "I'm dead"---"DG is dead, MKH lives on".