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8 Primarchs turned traitors and joined Horus in his heresy against the Emperor and his crusade.

The simple, global reason is that they were closer to the Warmaster than the Emperor and when Horus turned against his father, they decide to join him out of loyalty. But I don't completely buy it. Sure, loyalty and trust in Horus is part of the explanation, but Primarchs are depicted as extremely intelligent, with a high sense of strategy, tactics and logistic, and couldn't ignore that a rebellion would lead to a violent civil war that turns brothers against brothers and turn the Imperium into a wasteland. But many of them joined Horus without a second thought and seems to have no problem with slaying other Space Marines during Isstvan events.

The reasons for the treasons of some Primarchs has been explicitly given in the lore. For example, Lorgar believed that mankind needs faith and was disappointed by the fact that the Emperor refused to be considered as a god. His bitterness grew after the events on Monarch. He instigated the Horus Heresy in the name of the Chaos Gods. Angron hated the Emperor since they met, and Alpharius/Omegon decided to join the Heresy to fulfill the plans of the Cabal.

On the other hand, I have no clue about Mortarion. I have not read The buried dagger yet so I don't know much about his personality, except that he is not of the fun and smiling type. He had the classic "Primarch upbringing": raised in a harsh environment and helped freeing people of his home planet before being found by the Emperor.

It seems that he had no hesitation in joining Horus but no reason is really stated in the novels I read so far. He was a strong opponent of the use of psykers in the legions so it is a bit strange to me that he decided to join the champion of the powers of the Warp in his heresy.

Why did Mortarion decided to betray the Imperium and join Horus?

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    @RigaCrypto if I recall the sequence of events here, the OP raised this as "why did all the traitor Primarch's Turn", that was marked as too broad, so several have been raised individually. Mortarion and Magnus are very different characters, so I can't see the answers being duplicates. – Jontia Nov 11 '19 at 9:28
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    @Jontia My bad, read the same name in both questions by mistake and the overall text looked similar. Deleting the flag. – RigaCrypto Nov 11 '19 at 9:51
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    @RigaCrypto the copy-paste format of the multiple questions is a bit unfortunate. The OP could probably tighten up the questions a bit, make them clean and focused on just the Primarch in that question. – Jontia Nov 11 '19 at 9:57
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    Possible duplicate of Why did Magnus join Horus in his Heresy? – Chenmunka Nov 11 '19 at 10:04
  • @Chenmunka not really. Magnus' fall was quite unique. – OrangeDog Nov 11 '19 at 11:00
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Mortarion is strange in that of the chaos worshiping primarchs (Pertuarbo and Curze never worshiped the pantheon) he joined Horus long before turning to Chaos, where as Fulgrim and magnus had openly started worshiping pretty much as they joined the cause, Angron without openly worshiping Khorne was happy doing his bit for the god of violence. Mortarion joined the heresy because his feeling about the Emperor could be manipulated by Horus to present a vision of a galaxy where the strong rule the weak, and humanity learns to accept it's place beneath the power of the legions, not, as the emperor wanted the legions serving the people

Horus built on several points of Mortarion's personality.

First, Mortarion's hatred of his adopted father, the tyrant who lived on high unable to be attacked, who controlled the people living below him. Experimenting on them, using his powers to transform the humans into warriors created to fight for him and bring oppression. A being that used the powers of the warp and denied them to his children. Horus drew on the comparisons between the Emperor and his adopted father.

Emperor killing the Tyrant. Mortarion never really got over the fact that the Emperor went and killed the man he had fought so hard to try and kill all his life to that point.

The fact the Emperor wielded the powers of the warp himself, powers Mortarion originally saw as being evil, helped drive Mortarion to hate his father more.

Mortarion also believed in the idea that Might is Right so seeing humanity being given more and more control went against his base beliefs that the strong should rule the weak.

Finally Mortarion felt closer to Horus then he did the Emperor, he felt that Horus could provide the kind of galaxy he believed in and also realized that if Horus fell he might be the one to step in and take over.

Once the decision was made and he sided with Horus he started to realise the powers of the warp where influencing things, his fall to the warp came initially from him trying to understand the powers so he could destroy them and then finally his betrayal by his favorite son Typhon, who you learn from the Buried Dagger was with Mortarion on Barbarus from the start and may have already had dealings with warp entities, pushed Mortarion to willingly fall into the arms of Nurgle as, similar to Magnus, he finally realized his path had been set and he had been manipulated to this moment his entire life.

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