I like the idea that, when Fulgrim first starts sowing dissent, Corax tries to keep momentum going and redirect their fervor elsewhere. Like towards another errant brother. Like someone who has, since Ullanor, ignored Corax's commands and essentially set up a pocket empire. Jonson. Plan backfires, dissent sparks off into open rebellion. Corax convinces some brothers of helping him cement his claim to authority. Some jump aboard, some need some convincing, some need to be dealt with, some open fire.
Corax determines that Terra, housing Fulgrim and the Emperor's remains, is a threat that needs eliminating. And the galaxy burns.
Edit: Looks like I missed some replies.
Gotta admit, I don't see Corax as ever being loyal in truth. He might need to be rewritten a bit for that to show through.
Edited by Cormac Airt, 27 October 2013 - 12:32 AM.