Who's side are you on, Alpharius?
I have looked through the rules for the Primarchs multiple times over to get a better feel for them in the new Second Edition of Horus Heresy.
I have checked my main observation for this article three times to make sure I wasn't deluding myself. But here it is all the same.
Alpharius does not have an alignment.
What I'm saying is that he is not listed as either "Loyalist" or "Traitor". All the other Primarchs have the relevant alignment next to their special rules. Alpharius is the only one to NOT have which side of the conflict he is on specified.
Let me take this observation further.
Almost every named HQ choice in the army list also has an alignment. Garviel Loken (Sons of Horus) is listed as a Loyalist. Argel Tal (Word Bearers) and Ahzek Ahriman (Thousand Sons) are Traitors. The Lion is a Loyalist (I was considering making a joke here, but thought better of it).
Compare this to the other Alpha Legion HQ selections.
Armillus Dynat lacks an alignment.
Exodus similarly does not have it listed against his name either.
This is quite extraordinary. I would advance the opinion that this is a very deliberate choice / no accident on the part of the designers. The upshot is that Alpha Legion players can field an Alpharius commanded army standing side by side with other Loyalists. Welcome to second edition fellow Alpha Legion players.