Why did Perturabo join Horus in his Heresy?

Question

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. 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 treason of some Primarchs have 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 Chaos God. Angron hated the Emperor since they met, and Alpharius/Omegon decided to join the Heresy to fulfill the plans of the Cabal.

The main reason for Perturabo's treason is loyalty. At the beginning of Slaves to Darkness, Perturabo is shown to not be happy with what happens to the Traitor legions since the beginning of the Heresy. He is also upset by the high cost his legion has to pay in its war against the Ultramarines. He explains that he accepts to find an unruly Angron only by loyalty to the Warmaster.

But this begs the question: why not staying loyal to the Emperor then? The destruction of Olympia must play a part but I am not sure about the chronology of the destruction of Olympia and the Horus Heresy. The lack of appreciation of his actions during the Crusade made him extremely bitter, but he had no reason to believe that this would change after betraying the Imperium.

Why did Perturabo decide to join Horus in his Heresy?

Answer 1

Perturabo's weakness was his pride. he saw himself as being better then Dorn, a better seige master, a better designer of defenses and a better general. When Dorn was chosen to go back to Earth and be its defensive architect it angered Perturabo. When Perturabo found his legion being split apart, forced to garrison planet after planet, and received none of the glory or support the other legions received, it added to that sense of anger. His legion was expected to take out the strongest fortresses, enact seiges and battles that were brutal, bloody, and cost large numbers of his marines without any hint of appreciation from his brothers, or his father.

Horus used this, and he played on these feelings of neglect and anger at being passed over for the defense of Earth. He promised Perturabo he would be able to prove once and for all he was better than Dorn and he promised him the respect he deserved. Perturabo on his side felt immense loyalty to Horus as his Warmaster. But Horus also promised him forgiveness for destroying Olympia, an act that would have seen him censured and punished much as Curze would have been.