Some possibilities: 1) The traveller may be lying and we ARE the evil faction. (Highly unlikely though - bungie like heroes) 2) An evil human wanting to join the Darkness may be akin to an evil Jew wanting to join the [url=http://www.wikipedia.org/wiki/Godwin%27s_law]-godwinslaw!-[/url] - it's an issue of being, not morality. 3) There may be a story event down the road that creates a schism amongst the humans, and allows players to pick a side. 4) Ghosts might only select Guardians who fit a certain criterion on the morality spectrum.