People often like to discount the things that happen in the book because the information is presented by Xenos. The fact remains that the information presented by them shows us a galaxy where humanity would be badly mauled by its war with Horus and chaos, but ultimately there is a shot that we avoid extinction. The alternative is that Horus loses and we have the universe we currently have, a decaying mess of an empire that more or less enforces slave labor to barely maintain itself. The population lives under a decaying regime that has ultimate control of their lives while the high lords, mechanicus, and church run the imperium into the ground. All the while humanity's power in the galaxy is on the decline until it reaches a point where our enemies can deliver a clean blow that ends us for good. If various Xenos races are unreliable as far as info goes then sure, discount all of this. However I think that the Cabal realizes that if humanity were to become extinct then the only thing that actively works as an ork population control would no longer exist.
There are several elements that have to be questioned, to be honest.. 1) Did the Cabal present their information in good faith, or was it intended to deceive Alpharius? 2) Irrespective of the above, was the Cabal's prediction of the future actually correct (NB. The Eldar didn't get heavily involved in the Heresy - where they did, it was to attack chaos rather than support it)? 3) Did Alpharius agree with the Cabal because he was swayed by their arguments, or because he wanted them to think he was? Is he actually a secret 'loyalist', or did the Cabal's information push him to join Horus' rebellion for his own reasons? 4) Do the Alpha Legion know what Alpharius' plan may, or may not have been/was/is? If they did, are they still sticking with it, or after 10,000 years do they now follow chaos and oppose the Imperium for their own reasons?
Read the book again, there is nothing said about avoiding destruction, it is the damn goal of the whole horus wins thing.
I think the Alpha legion know what it is doing. They know that humanity is doomed no matter what. We win the battle, but become playthings of chaos. Slowly losing what makes us human. Or we die on our feet and take chaos with us. They are probably trying to make Abaddon win the long war, but making it take time and effort. In hopes that his hate of humanity will grow and drive him to kill us all.
I believe Kanthric sums up the confusion fairly well, part of it is caused because the ending of "Legion" which should have enlightened us on this, was rushed. You spend this whole novel on a war that in the end doesn't matter and then have the primachs turn traitor in about 5 pages after being insanely suspicious previously. Some extra time spent on their logic would have helped. Also no one seems to think that the instant you know the future it can be altered, the Alpha Legion could have taken that info and informed the Emperor, or attempted and assassination of Horus. The Cabal failed to show why NO OTHER OUTCOME could defeat Chaos, because it seemed the Emperor had a plan that was unknown to the Alpha Legion. TLDR: The Alpha Legion made a hasty decision without all the facts, I guess then they were stuck with it.
I always assumed that Alpha Legion would put their own spin on Cabal's plans as is their custom (We could see Omegon already rebelling agianst Cabal in Deliverence Lost). I think they believe, what cabal says is true and would do everything to help Horus win the battle. After the victory they would simply leave the galaxy and let it burn itself while the whole legion and their followers go into hiding. In the end they would end up as the only humans left in the galaxy and have knowledge of chaos to hopefully keep it in check. However, since Horus can't do anything other than using his genius "Spearhead" tactic, they lost (Even Dorn said before Battle of Terra, if it was Alpharius or Curze that lead traitors, they would eventually win). So after that point it was too late fo AL to go back. After 10000 years I don't think the original plans hold true anymore since AL is spread through out the galaxy and each head works seperately. So other than Omegon (or Alpharius or both?) playing the longest f-ing games ever, alpha legion is full of divided goals. Some might still know of the original goal, but I guess we will never know.
I just came to realize something as I was rereading Legion. Think about this quote. "We have come to appreciate that Utopian goals are ultimately counter-intuitive to species survival. No power can engender, or force to be engendered, a state of perfection. Because perfection is an absolute that cannot be attained by an imperfect species. It is better to manage and maintain the flaws of man on an ongoing basis." And sins the Cabal is made up by imperfect species, then their goal to kill chaos with mankind is Utopian. And therefore cannot be attained. But to manage and maintain the flaws they must make sure that Horus do not win and they need to keep an eye on chaos. So the Alpha Legion will do this as they always have done, in the shadow and behind enemy lines.
