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 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.
Primarchs must have reasons to join the Horus Heresy. What are the individual causes (personal reasons, or reasoning) that pushed each Primarch to join the Heresy?
The case of Magnus eludes me. At first, because of the events with the Golden Throne and the subsequent events on Prospero, I believed that Magnus' fall to Chaos was independent of the Horus Heresy. At the beginning of the raid of the Space Wolves on Prospero, Magnus seemed to regret his actions and it is not until Arhiman pushed him to action that he decided to save his children and flee to the Sorceror's Planet. To my surprise, Magnus appears at the end of Slaves to Darkness and shows total allegiance to Horus. He can certainly not come back to the Imperium but I don't really understand why he would follow Horus in the destruction of the Imperium. He knew that Horus' rebellion would lead to a catastrophe for the Imperium and mankind, since he tried to warn his father of the treachery of the Warmaster. He certainly knew that one cannot deal with the powers in the Warp without paying a high price.
Note: I read about Magnus' shards but I don't really understand this part of the lore.
Why would Magnus decide to join Horus in his Heresy?