There are tons of security cameras throughout the tower.... Why? Are the guardians not to be trusted? I've heard all kinds of theories, my favorite being that the fallen were like humanity before. Blessed by the light if the traveler, up until something happened, and they were cast out of his light. Now they follow after him, hunting/plaguing his new children, trying to gain back favor with their old God/deity. The hive are the true darkness (now that the original storyline has changed completely). Like a plague moving through the universe, enveloping anything, because their existence is tried, true and wins against everything else. Much like the Vex. I believe it was Toland the shattered who fretted about this constantly in the grimoires. Queens/armies that existed because their formula for existence prevailed over others, which perished and ceased to exist. This "working" formula was passed on, whatever list was not. This is how the Vex grew as well. Evolving to the point where they could control space/time, and even existence. They are not the darkness, something totally different, but key in the big picture. The cabal? They're another species irrelevant to the story, but there to add another degree of depth/something to kill. Or they're simply after the traveler, its power and our planets resources. Humans stumbled across the traveler, it viewed mankind as another force capable of helping it fend off the darkness, and helped bolster mankind in order to do so.