Some of the Primarchs went over to Chaos, because the Emperor ordered them to. The Emperor wanted to get rid of Chaos and to do that he'd probably need agents on the inside.
Horus was the most unlikely to betray the Emperor, so clearly he must have been ordered by the Emperor to join Chaos when given the opportunity.
Magnus, being a really powerful psyker, was prime bait for Tzeentch. The whole raising of Prospero was arranged between Emps and Magnus, so that Tzeentch would take him in.
Night Haunter was not meant to be let on on the secret, but he had a vision and so the Emperor had to explain things to him.
Several other Primarchs had been groomed as bait for the Chaos Gods, hence why Emps acted like he did towards Angron, Mortarion, etc.
Am I doin it rite, guise?