I think that after 10,000 years, the Alpha Legion is undoubtably for Chaos.

However after reading Legion *spoiler alert* I think that Alphariaus orginally sided with Horus with good intentions. However, with his death at the hand of Guilliman, I don't think anybody else in the Alpha Legion legitamately knew why Alphariaus sided with Horus, so know they're just embroiled in Chaos for Chaos' sake. Even though Omegon still lives.