Board Thread:Warhammer 40k General Discussion/@comment-67.197.44.118-20130216115719/@comment-2253300-20130301150140

Because they originally turned to Chaos in order to destroy it. They believed that they were doing the Emperor's will. However over the years I believe that they have gone fully traitor. One cannot spend so many years in the service of Chaos and not have your agenda twisted. Also it may be that their battlecry is their way of mocking the loyalists.