Oh - and just in case the "Russ was sent to get Magnus before Magnus did anything because the Emperor knew he was going to do something" idea should be regurgitated - in the same scene as Russ refuses to believe Horus is a traitor, he knows that the reason he's been sent to take out Magnus is that he had made that claim during his sorcerous message. He's not been sent there because Magnus is going to use sorcery, and Horus turned traitor while he's en route and he didn't know about it; he's been sent there because Magnus has used sorcery, which (according to TOD) only happened long after Horus turned traitor... in which case Russ couldn't possibly have still thought Horus loyal.