I don't think that Bannor/Order society really allow for the existence of Political rivalries, but the Overcouncil certainly does.
To me it seems more likely that he started out Malakim, and studies under a famed Empyrean Philosopher (cf. Alcibiades study under Socrates). He was probably more politically than philosophically skilled though, and used his mental prowess and rhetorical skill honed in discourse with the masters to gain political power instead of emlightenment. I see him as becoming a dominant personality of the Overcouncil during time of war against some evil, and/or perhaps the Commander of the Joint Forces of the Overcouncil's member civs. He loved war and (especially) his position of command more than the principles of Truth and Justice that the Empyrean/Malakim held dear. Some evil civ tried probably to surrender. The Malakim/Empyrean wanted to show mercy, but Bannor/Order didn't trust the terms of this surrender. This split the council, and may have lead to actual combat between its member civs. Decius saw his power depended on war, and so betrayed his desert people to lead the Bannor in their unrelenting crusades. While the Bannor may have been right and their enemy unrepentant, I suspect this was during a dark period of Bannor history. This is probably when the Church of Junil was corrupt, and devoted to hunting down all true believers. Decius was probably important in this persecution. He probably convinced the Calabim to join The (corrupted) Order to aide their conquest of righteous peoples. In this period, the Bannor were beginning to take their orders from the Undercouncil. Eventually, this corruption was revealed and Decius was deposed by Valin Phanuel (or his followers, maybe after the leader was martyred). Decius then fled to his Calabim allies, and was probably eventually given "the gift."
There may be more chapters in this story though, perhaps involving being a double, triple, or quadruple agent for the Overcouncil and Undercouncil. Perhaps no one really knows where his allegiance truly lied. He was probably trying to play both sides.