The whole betrayal swings around Chaos forces manipulating the arrogance and pride of the Primarch's. Horus felt betrayed by the Emperor because he was returning to Terra and turning over administration of the Legion's conquests to " humans" and not to the Primarchs. Furthermore, Horus felt that the space marine legions were going become "policemen" instead of remaining the god-like warriors they are!?? This was the beginning of the rift between Horus and the Emperor. Even though he was Warmaster he felt deserted by the Emperor.At that moment it became easier for Chaos to seduce him. It was this rift that Erebus used in Horus's dreams while recovering from his wound to turn him. Also, Davin was a Chaos controlled planet and Erebus's legion on the most part was already in Chaos's clutches. The rest of the Primarch's that came over to Chaos were each seduced through their simple desires weaknesses. They may have been super-human, but still contained all the human weaknesses(pride, arrogance, jealousy, ambition,etc...) These were all used masterfully to get them to follow Horus and the forces of Chaos. The legions followed their Primarch's willingly, but were also seduced through their own pride and ambitions. They resented that the Emperor had returned to Terra and turned administration over to regular humans and that the "Great Crusade" was coming to a close. They were ripe for the rebellion to come. Chaos (with Erebus's help) played on their discontent and the Heresy came to life. The Primarch's willingly followed their own desires and went with Horus into war against the Emperor and his forces.