Bear in mind that this book is my very first foray into the Warhammer universe, and I enjoy it a good deal. That said, I'm not entirely convinced I understand the rationale behind Horus' fall.
Take, for instance, his dealings with the Interix, and this exchange with the Mournival in relation to diplomacy with them:
The Warmaster, it seems to me, is perfectly correct in his elucidation of the situation: the Interix has no innate conflict with the Imperium or the Emperor, and, it seems to be implied, mostly wants to be left alone. But Imperial protocol would seem to deny them this possibility."Two weeks without solid agreement," Horus said,"'nor even a mutually acceptable scheme of how to continue. They regard us with a mixture of curiosity and caution, and hold us at arm's length. Any commentary?"
"We've exhausted all possibilities, lord.’ Maloghurst said, 'to the extent that I fear we are wasting our time. They will admit to nothing but a willingness to open and pursue ambassadorial links, with a view to trade and some cultural exchange. They will not be led on the subject of alliance."
"Or compliance." Abaddon remarked quietly.
"An attempt to enforce our will here.’ said Horus, "would only confirm their worst opinions of us. We cannot force them into compliance."
"We can." Abaddon said.
"Then I'm saying we shouldn't." Horus replied.
"Since when have we worried about hurting people's feelings, lord?" Abaddon asked. "Whatever our differences, these are humans. It is their duty and their destiny to join with us and stand with us, for the primary glory of Terra. If they will not..."
He let the words hang. Horus frowned. "Someone else?"
"It seems certain that the interex has no wish to join us in our work." said Raldoron. "They will not commit to a war, nor do they share our goals and ideals. They are content with pursuing their own destiny."
Sanguineus said nothing. He allowed his Chapter Master to weigh in with the opinion of the Blood Angels, but kept his own considerable influence for Horus's ears alone.
"Maybe they fear we will try to conquer them." Loken said.
"Maybe they're right." said Abaddon. "They are deviant in their ways. Too deviant for us to embrace them without forcing change."
"We will not have war here," Horus said. "We cannot afford it. We cannot afford to open up a conflict on this front. Not at this time. Not on the vast scale subduing the interex would demand. If they even need subduing."
"Ezekyle has a valid point.’ said Erebus quietly. The interex, for good reasons, I'm sure, have built a society that is too greatly at variance to the model of human culture that the Emperor has proclaimed. Unless they show a willingness to adapt, they must by necessity be regarded as enemies to our cause."
"Perhaps the Emperor's model is too stringent." the Warmaster said flady.
There was a pause. Several of those present glanced at each other in quiet unease.
Bear in mind that this is literally the first material relating to this universe that I've ever come across, and I haven't finished it yet. That said, I have to ask: is this mere disagreement, and other disagreements of this nature, between Imperial doctrine and the Warmaster the cause for his eventual turn to Chaos? If so, I have to say that I think his argument is entirely correct, and the Emperor's creed entirely wrong. Being that this is years before the need to stamp out the possibility of corruption, I can hardly see the need to exterminate an entire population of humans merely because they are not culturally pre-conditioned to accept Imperial dogma.
The wise thing to do, it seems to me, would be to issue an apology for intruding upon the megarachnid planet, leave behind a few remembrances and iterators for cultural exchange, and leave their territory and continue the Crusade. But I get the feeling that isn't what happens.
