Well my own theory is that the Vex are the only real enemy to humanity. If the Darkness is converging on earth from all directions then it would seem logical that all other species in the universe would converge on earth seeking refuge from the Darkness. Also if you read some of the cards on Rasputin then it becomes clear that HE was the one that cause the collapse in a calculated sacrifice of enough human lives so he could harness their souls (a powerful source of Light) to push back the Darkness. I also theorize that he was the one that created the Ghosts from these souls and not the Traveler but that's just me. Throughout the entire campaign WE are the aggressors. We assault the Fallen, Hive, and Cabal but not once do we defend against an assault from them. They are just like is seeking answers and information to defend themselves against the Vex and the Darkness. A prime example is the mission on the moon where the hive are "draining" a shard of the travelers of it's light. Could they not be doing this maliciously and just trying to harness this power for their own protection? I think it's a good possibility. Just my idea anyway, take from it what you will.