Board Thread:Warhammer 40k General Discussion/@comment-92.97.77.182-20130321145115/@comment-2093048-20140509025558

Some things of note:  THere have been other books since Legion. So don't use Legion as the final source regarding the Alpha Legion's loyalties. Other books clearly show taht THEY BETRAYED THE CABAL, too.

Hell, they find a way to screw over EVERYONE whenever they appear. The Cabal (Deliverance Lost), Horus (The first battle of Paramour), the loyalists in general (Ishtvaan, which some believe was planned by them, and whatever else), and even Alpharius for crying out loud (Yes.  Alpharius.  The short story from The Primarchs" anthology)

So don't use Legion as an example of them being loyal. Because Alpharius showed that he's willing to betray the Cabal too for his own ends. Ends which apparently are in conflict with his own twin brother despite how the two supposedly share the same soul.

As an aside, there are several Daemon Princes that come from the Alpha Legion, so I'm sure at least some of them are in Chaos' thrall.