I believe, the Fallen were a race once blessed with the Traveler's light just as we are. But this caused civil war among them and warring houses and later the Traveler abandoned them. They are seeking vengeance. The Cabal are militaristic and interested in Rasputin and Warmind tech solely for warfaring purposes, I believe. The Hive are an evil and just taking advantage of our miserable situation. The Vex, they keep certain things locked out of time. Why? To protect, I think, from the Traveler. Keep in mind a Warmind is the only thing that can out-think the Vex, and they worshiped the Black Garden's heart, something that they didn't understand.