This is just a theory... The fallen are simply bandits, they are suprisingly against the darkness (which is why that dreg screams "the darkess") they are fooled into believing you are part of the darkness, becuase the traveler wants to eliminate all opposing the chosen ones, but to skeptis prime or the archon priests, does one not agree that they are the chosen ones in that perspective, the hive are obvious darkness, and so are the vex (have you ever seen the vex and the hive in a battle?) the cabal want to eliminate the darkness too, which is why they attempt to find their way into the black garden, and kill their God, but they want to kill us too, so are we part of the darkness? This game is very metaphorical, for the inevitability of war and the everlasting power struggle of teams, the everlasting fight. This is a bit of a strech, but could that be why you can do the same strike twice? Everything in this world is war, we are in a wedlocke with war, and we will continue fighting until the rest of the world is dead, which is the exact same motif as every other alien race... So who is the darkness? Where does false preseption end and reality begin? We were born with these intentions as gardians, but so were the hive, and the vex, they were born into these intentions, so who is in the wrong?