Kriswall wrote:
During the Heresy, the Alpha Legion were essentially asked to make a decision. Down one path, humanity destroys Chaos, but goes extinct in the process. This is what the Eldar/Cabal want to see occur. Down the other path, humanity lasts basically forever. To get down the second path, the Alpha Legion have to turn against the Emperor's wishes. They believed that the Emperor would want humanity to last, so they made the decision to fight against him. It's an example of the ends justifying the means. They martyred their loyalty to achieve what they believed to be they're boss's long term goals.
I think this highlights the real thing that's a sticking point here. The fluff (at least the stuff I've read, like
Legion) never really gave me the impression they were "faking" their fall, or somehow tricking the Chaos gods or working as double agents. It more implied that they were led to believe that staying true to the Emperor would bring about disaster in the long term, while siding with Horus would lead to the better long-term outcome for the Imperium, so it was their duty to do the latter. I never got the impression that they were planning to come out at the end and go "Ha ha, I was only pretending to work with Chaos!" They knew they were selling their souls to the devil, they just thought that the outcome was worth their sacrifice. Like if you lived in Germany in 1930 and had a psychic vision of the future and decided you needed to assassinate Hitler; you're going to go to jail for murder (or a loony bin for your psychic visions defense), but you might judge it's worthwhile anyway. (Also I realize this is basically the plot of
The Dead Zone.)
So I think the answer is really that their heresy was out of a sense of loyalist duty instead of arrogance or greed or bloodlust or whatever, but that doesn't make them any less heretical. They just judged that it was worth the price. They'd have made great Tau.