Originally Posted by
Katharon
One of the defining characteristics of Space Marines is that they don't deal well with failure. They are suppose to be (and are) super human and beyond the failings of mortal men. As such, when they fall or fail, they do so in a much grander fashion than normal men and women. Cyrene is, you could say, Argel Tal's albatross.
Cryene was one of the few individuals saved from Monarchia when the Ultramarines destroyed it on the Emperor's orders. Her survival, and loss of her sight, turned her into a sort of lucky charm and a reminder to the Word Bearers. She was a living survivor and example of what they had failed in the eyes of the Emperor. Since they are quite easily swayed into devotional thought processes, she became a token. Word Bearers could speak to her freely, like modern men speaking to a Catholic priest for absolution. She would accept them without judgement, allowing them to purge their own doubts by expressing them to her. That kind of tool or ceremony cannot be underestimated for a fanatic.
Cyrene also became the closest confidant of Argel Tal. It might even be fair to say that the Space Marine was a little in love with the woman. Argel Tal decided that if he could protect anything ever again, then it would at least be Cyrene that he protected. It was one of the reasons why he had the Mechanicus Cybernetica engine left behind to act as her protector.
Perturabo's reasons for betraying the Emperor are more complex than that. The foundation of his betrayal was indeed built upon the feelings of under-appreciation, becoming the guy that does the dirty work that no other Legion will take. Horus used that as the stepping stone to other emotions within Perturabo. When his homeword of Olympus rebelled, one of the first times any Primarch's homeworld had ever done so, the mere shame of it enflamed Perturabo.
How fit for leadership or responsibility could a Primarch be if his own homeworld rebelled against the Imperium? So Perturabo brought his Legion back to its home and the Iron Warriors did more than supress the rebellion. They murdered a whole world. Olympus was destroyed in all but name, its populace massacred before the bolts and blades and treads of the Iron Warriors. It was an act of such barbarity and slaughter than Perturabo could not bring himself to face the Emperor over it. He knew that the Emperor could never forgive him for slaughtering Olympus -- that he and his Legion would face censure at the very least, just as the Thousand Sons had been punished by the Space Wolves.
Horus took advantage of that sorrow and pain. He came to Perturabo as an understanding brother; giving him the forgiveness and absolution that Perturabo needed, though would not admit to needing. Two hundred or more years of being treated as the Legionnes Astartes cleaning boys, the irritation of never receiving their just recognition, and the shame of having perpetrated one of the worst crimes a Legion could perpetrate led the Iron Warriors along the path of perdition.
Angron is more than that. His reasons for betraying the Emperor are probably one of the most "just" if you care to look at in one way. His reasons are certainly personal. The slave army that he had created, his companions that he called brother and sister, was the most dear thing to him in the universe. The Emperor, for reasons we do not know, chose to extricate Angron from the final last stand that his slave army would have made -- but did not make an effort to save the slaves themselves.
So the Emperor did the following, in Angron's eyes:
1) He took away Angron's choice of dying alongside his slave brothers and sisters.
2) He denied him the right to even witness their deaths or for them to understand that he had not abbandoned them.
3) Even though it would have been in his power to do so, the Emperor did not save those slaves that Angron called brothers and sisters.
4) The Emperor retained the highrider government that had enslaved Angron in the first place and left it to continue ruling over that world.
Those reasons, when Angron is not taken by the Nails, are why Angron hates the Emperor. The Butcher's Nails in his head only help to fuel that anguish and hatred to further heights. From any perspective of how the Horus Heresy could or would have gone, eventually Angron would have either died stupidly in combat due to the Nails taking his senses away from him or else the Emperor would have been forced to fully censure him as had been done twice before to two other legions.