I found a Grimoire card that is called "Legends 2". This card talks about The Nine.
In one part it says: "The Nine are the aspects of the Darkness, broken by the Traveler's rebuke, working to destroy us from within."
So if Xur is an Agent of The Nine, doesn't that mean that Xur is working for The Darkness?
Either way I'll still be buying exotics from him.
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They are the small light in the dark we are the rest