The traveler is both the light and darkness. Xur states i believe at one point that "the traveler has a dark mirror" or something of that effect They are both and the same. That is why the traveler sacrifices himself. If one dies they both die. Hive are given power from the darkness the same way the Guardians gain power from the light. I think the traveler created all races, but humanity. To create a perfect race, only things went bad..... the first of each race was killed. To resurrect the old gods, these races saught after the traveler for the power to bring them back.