Light casts shadows, shadows are dark. Darkness always follows light. Without light, there is no darkness but with too much light the darkness has nowhere to thrive. A paradox but it also allows us to see the darkness' true nature. To control the light so that the darkness can never fade. Also the fallen are simply butthurt that the traveler abandoned them when it saw that they couldn't fight the darkness, it probably would have left us if it wasn't damaged. Probably in the reef wars or when crota took the moon