The Emperor needed a weapon that would never obey its own desires before those of the Imperium. He needed a weapon that would never bite the hand that feeds. The World Eaters were not that weapon. We've all drawn blades purely for the sake of shedding blood, and we've all felt the exultation of winning a war that never even needed to happen. We are not the tame, reliable pets that the Emperor wanted. The Wolves obey, when we would not. The Wolves can be trusted, when we never could. They have a discipline we lack, because their passions are not aflame with the Butcher's Nails buzzing in the back of their skulls. The Wolves will always come to heel when called. In that regard, it is a mystery why they name themselves wolves. They are tame, collared by the Emperor, obeying his every whim. But a wolf doesn't behave that way. Only a dog does. That is why we are the Eaters of Worlds, and the War Hounds no longer.