Its complicated.. Short Answer in my opinion: No, they are not loyal. They wanted to end chaos/save mankind, but they did so by defying the Emperor. Which is traitorous. Even if you asked instead: "Are they loyal to the Imperium?" The answer would still be no, as they actively fight against the Imperium. If you asked "Are they loyal to the Chaos Gods?" Then the answer would be: Yes, no, and maybe, all at once. The legion as I see, is now a symbol of Tzeentch's craftsmanship.
Unknown. During the Horus Heresy, the had the plan to destroy Chaos by sacrificing mankind. they openly fought against the Imperium while secretly sabotaging Horus. Their goal was that the forces of Chaos would destroy each other in a civil-war right after their victory over the Imperium, saving the rest of the galaxy. Well, that was the plan... The later HH-novels complicate this even more. It's not spelled out, but... Alpharius is described as enjoying the destruction he causes and Omegon begins to secretly sabotage the Alpha Legion's efforts to weaken the Imperium by sabotage... for whatever reasons. We don't know what happened to the Alpha Legion at the end of the Horus Heresy. Novels situated in the years and centuries right after the HH (Talon of Horus, Kharn, the Ahriman-novels...) tell practically nothing about the Alpha Legion. They are present in the Eye of Terror and they took part in the Legion Wars. And Abaddon's newly-formed Black Legion defeated all of the other traitor-legions, forcing them to stop fighting against each other and unite under his command for the 1st Black Crusade.
I don't even think Tzeench himself knows the answer to this question... Imo it's probably a three part split. A faction of splintered warbands that has fallen to Chaos, a faction that maintains steadfast loyalty to the Imperium and the Emperor, and a faction that has its own agenda whether that be the Cabal or otherwise.
They probably don't actually exist I mean, come on Who can believe there's an actual legion that honestly believes the best way to save humanity is to destroy it? It's clearly troll work of Tzneetch
Thanks for all the replies everyone, it's interesting to see the diversity of perceptions about the Alpha Legion.
Now what tickles my curiosity is why haven't they fallen into Tzeentch worship... or how often do Alpha Legion marines do.
