User blog comment:Primarch11/Guilliman Heresy/@comment-3245042-20120615152958/@comment-3258762-20120615155649

although, Supah, they sort of need something like the Burning of Prospero to push them. They turned traitors when their world died and they escaped into the warp, where Tzeentch constantly, pretty much, ticked them off about loosing their homeworld, which led Magnus to become bitter over the Emperor's order that he be destroyed.