I've been saying there is a cycle to this game for a while. The cutscene with Ares One has changing mountains in the background, implying the discovery of the Traveler has happened many times. Bungie loves to reference their other games, and this would match Myth. A hero rises to topple the evil cycle, and begin one of good. By the end of the good cycle he becomes the new evil for the next. Almost sounds like Sword Logic, doesn't it? Killing Oryx and other Dark entities only further pushes our character to take their strength and their place.