The theory of Traveler=Evil isn't fairly new but definitly posses some truth in it. Allow me to present a more grey-ish thoery. First we need to consider each specie we encounter as "factions", what I mean by faction is just the regroupment of individual or some can say "sides" even maybe alliances. In my opinion there is two main sides and several smaller ones. We have the Traveler and the Darkness which seem to be entities "helping" forms of life to progress in the universe. The Traveler appears to be a sort of "technology provider" and help species which are sufficiently developped to reach their full potential (if not, why bother waiting on Mars to be reached by three humans?) by providing marvels and new techs and biological advancements. The Darkness, in the other hand seems to prefer a more "brutal" way where selected species have to fight against other existing species in order to become stronger, wiser, and more advanced. It applies very well to the Hive which followed this kind of path according to Toland (There is a Grimoire Card describing the process, sorry for not providing it). The Darkness doesn't have a particular quarrel with the Traveler, it just follows its plan and sometimes those two philosophies collides. And Falls happen when it's the case. We only call it Darkness because it is aggresive toward us and the Traveler, that's just the name we put on it. According to the Grimoire and Variks, the Fallen were a specie helped by the Traveler in the past but when the Darkness came with its species (maybe the Hive), due to its more pacific side, the Traveler left (Kind of a Dickmove if I may say). The Fallen tried to fight against the Darkness but were finally forced to run. They decided to follow the Traveler in order to regain their power and maybe defeat the Darkness. Due to the travelling time and other unknown circumstances, they reached our system during our Fall. Since then, they want the Traveler back and think they have to prove they are worthy of the Traveler again (remember what I say about waiting to be found on Mars? Maybe that was when WE proven to be worthy). So, why the Traveler didn't runaway once again when the Darkness finally arrived in our system? We didn't let it go. More specifically, Rasputin and maybe the other Warminds used force against the Traveler in order to avoid its leave.(as far as the actual theory goes, Dickmove on our part) That's why the Traveler is so weak since the Fall, that's why he created the Guardians and the Ghost. Knowing that it'll not be able to leave in time, it created something to protect itself and make sure it can survive, and us with it by extension. And since the Fall we kept this kind mutual benefit situation until we can retaliate or push back the Darkness. All this means that Humanity and Fallen are more or less on the same "side", the Traveler's side but we just don't like each other or don't understand each other because, after all, we are different species with different ways to think. The Hive in the other hand seems to be the actual Darkness "champion" specie, the one it try to make evolve by "fire". But what about the Cabals? The Vex? I call them the Neutral Species or the Collateral species. The Cabals first, they're fleeing the Hive. It was not clear until the Taken King annoncement but in the Grimoire it's several times implied they are running away from something terrible. But they don't appear to be in relation with the Traveler either. Maybe they never crossed path with it or never proven to be worthy of it or whatever. They just are an alien specie which evolved by itself without be influenced by a "faction" and are currently trying to survive. They have now a particular interest in the Vex technology and/or the Black Garden maybe because it is sometime described as a place for protecting species? They also deeply want the Golden Age technology in order to retro-fit it and find a way to save themselves. The Vex are the most mysterious of all. We don't know when they appear, where and how. We just know that they can transform planets in machines, guard the Black Garden and are able to time travel. They were following their own agenda until the Darkness arrival. Here, I'm about to throw the most speculative part of my theory so brace yourselves. We know that the Darkness has interest in promising 'combative' species. What could be more interesting than a specie capable of time travel? So the Darkness invaded their 'home', the Black Garden and used its force to make them "workshipping" it. Maybe the Vex were following antoher goal like terraforming all the system like Mercury in order to totally be independant of Time, maybe the Vault of glass is a prison where the "rebels" against the Darkness are kept, who knows? But what is very strange is the form of the Heart in the Black Garden, a sort of moving shapeshifting orb which can sometime "look" like the Traveler. Hence, people saying that there is an "evil twin" Traveler or even that the Traveler is Evil in the future and such. But if you listen closely some of Variks lines in the Reef, one of it is "Would you like to hear the stories? The stories about the Shape Shifters?". Can it be the Darkness form? A shape shifting entity capable of deceiving other species? Shape shifting is a very good survival strategy, juste look at cephalopodes or cuttlefish. Here is an even more crazy theory: the Vex were tools/specie designed by the Traveler long time ago (for what? I don't know) and capable of time traveling. Upon its arrival, the Darkness discovered their existence and became interested in such a specie. It decided to "acquire" it by deceiving them and shape-shifted in a false Traveler. The Vex, seeing their "God" return, decide to obey and start following its instruction. You want a crazier theory? Here it comes: We beat the Hive in the future and the Darkness decides to find another specie to support/help and Oh look! the Vex! let's take control of them and travel back and forward in time in order to avoid the defeat of the Hive and hence, controlling two very powerful and advanced species! Fewwwww! Sorry for the long post and I appreciate every feedback about this theory.