Angron and the Emperor, predestined to be a fail meeting?
I just had the though that the main reason for the Emperor's success in swaying humanity to his cause was the psychic glamour of awe and greatness that he constantly projected, that sent men to their knees. Weeping or positively going blind from the radiance of his aura. Not even the primarchs where imune to it, getting taken in by his grand plan to reunite the disparate strands of humanity in a grand Imperium under his enlightened rule.
However damaged by the psyker inhibiting nails, Angron did not have the psychic aura of awe like his brother primarchs that made their space marine sons want to go through fire and water for them, leading to the world eaters desperately mutilating themselves with the nails in the hopes of getting just a glimmer of the awe and devotion that existed in the other legions between Primarch and their astartes progeny.
Then I hold, after reading Lord of the Red Sands, that the Emperor's psychic glamour simply did not work on Angron at all, and instead of seeing the saviour of humanity. Angron saw just yet another petty tyrant using everyone as tools for his own inscrutable goals, and why he choose to turn. He did not stand with Horus, but he stood against the Emperor.