Could it be the the enlightenment the traveller brings actually is changing races to be evil? Granted it could be with the best of intentions, however it still fails...maybe that's our destiny. For example time travel is something real in Destiny, what if the traveller has come to us in multiple timelines, in one such timeline the faction wars never ended do the fallen houses not reflect the factions of the tower? Another is The hive were not always the way they are now, eris was obviously changing as well as what we know form the book of sorrows and worm consumption. During the golden age mind was mixed with machine could we have made/become the vex?