So I'm going to briefly go over a few assumptions I am making in order to simplify my theory. 1) In star wars there is the force, period. Dark and light is simply a matter of how it is wielded and why. In Destiny it is a similar concept but in this universe they are two separate entities that are closely connected but not the same thing. 2) This is the key, the light and dark are not from our dimensions per-say instead they are from other dimensions that are closely linked to ours. 3) All beings posses some connection to these forces from other dimensions both dark and light, only those gifted or chosen can tap into it and wield it. So here is the theory, both dark and light can only be used by entities who resemble a living thinking, individual. The energies are directly connected to their life forces like a soul if you will. The Traveler is essentially the embodiment of pure light and it is at war with some kind of similar entity from the dark. It was losing its war and as such changed its strategy to seek "allies". It found our civilization and instilled in us powers and knowledge to propel the golden age. In truth what it trying to achieve is essentially a zealously devoted armada to combat its own enemies and such bring its war directly to our door step. However the traveler was still going to lose so it realized it needed more energy. The best way to get this energy was to absorb it from living beings at their death before the darkness had a chance. In steps the Guardians, designed specifically to kill and absorb the life forces of everything else. This energy is partially gifted back to the guardian to increase their power and push them ever further but most of it is absorbed back into the traveler. Evidence? Guardians don't defend, they annihilate. A single immortal soldier with incredible power can wipe out entire armies killing thousands with minimal effort and little to no chance of being killed. At the same time these guardians are specifically heading wars that put them in direct confrontation with entities that herald the darkness. Two birds with one stone as they say. So what does that make you? You're the charging cable providing light energy to a a being nearly beyond compare in power. You're entire purpose is to end life, over and over and over again to fill a bottomless cup. In a war that had nothing to do with you or your people until it involved you. But like all Guardians you have a drive to always push for more power, more light, a need, a craving that was placed within you by your almighty savior itself.