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[Warhammer 40K]Why did warmaster Horus rebel against the emperor?

I'm reading through the books and though he seems to suffer under the weight of his duties, but his reversal to turn the crusade for unification in to a rebellion seems to come out of nowhere.
gpt
When the Emperor ducks out of the Crusade to (secretly) work on the Imperial Webway project, that doesn't sit well with Horus or some of his fellow Primarchs, who feel they've been abandoned by their father. He was already angry about the fact that the Emperor had delegated a lot of authority to the Council of Terra, whose policies regarding the newly reclaimed human worlds Horus disagreed with.

When Horus is wounded by a Nurgle-infested blade and medics are unable to heal him, Erebus (a chaplain who is secretly in league with Chaos and a Marine who has become one of closest advisers to Horus) advises that they take him to a secret lodge on Davin where rituals are performed which cause Horus to commune directly with the Chaos gods. It should be noted that Erebus and the Word Bearers he served with had been severely reprimanded by the Emperor previously for trying to establish the worship of the Emperor across many of the worlds they conquered, and that this rejection had let them to align themselves with the gods of Chaos, who they viewed as being genuinely divine and worthy of humanity's worship.

They show him a vision of the future where the Imperium has become a brutal theocracy where the Emperor is worshiped as a god. They insist that they have little interest in the affairs of mortals and that they feel threatened by the Emperor's psychic power. They play on Horus's emotions and feelings of abandonment, and this drives him to reject the Emperor and turn traitor.