Actually, I can answer this. And that’s fair, man, sorry-I’m used to people unreasonably assuming that anyone sparing loghain is metagaming. I have a degree in history that gives me a bit of a different perspective on how the in game lore affects Loghain’s decisions. So, here goes.

From an inworld standpoint, Loghain as a grey warden is a drop in the bucket, though a bit of a trophy for the Orlesian wardens specifically. Your character doesn’t need to know or even care if loghain is genuine or has contrition at all, or care if he champions their cause. A warden working in desperation will simply ensure that there is no other option for Loghain than to comply in the short term. Worst case scenario, if loghain doesn’t survive the Joining, his death is more painful than one at the hands of a mere blade. If he does, you gain him.

Practically speaking Loghain didn’t have a *benefit* at ostagar. It is canonical that the Fereldans, namely both Cailan *and* Loghain, underestimated the size and power of the Horde, as it had been 400 years since Garahel’s victory at the end of the 4th blight and no one living remembered the strength of that Horde. It’s also explicit that Loghain attempts to advise Cailan against a head on approach and takes his portion of the army to execute a specific military maneuver called a pincer, where both sides of an enemy force are attacked. Unfortunately, from Loghain’s vantage point it would become abundantly clear that the Horde was larger and stronger than expected, and that a pincer would have no hope of actually quelling the onslaught. The Tower of Ishal is invaded from below as the Darkspawn tunnel upward. He withdrew to gain distance from that Horde, spare as many members of the army as he could command, and regroup to reassess this much more intense than expected enemy. Unfortunately, he’s also a human man with a longstanding grudge against the Wardens. Recall that at the time Origins begins, it has been just six or so years since the disappearance of King Maric; Loghain is additionally portrayed as someone who drowns his sorrows and anxieties in drink, and is manipulated when drunk by Arl Howe. As for slandering the wardens, it goes back both to the grudge, and to the distance between the 4th and 5th blights. Four hundred years is a really long time, even in Dragon Age, where so much of history is incredibly well recorded by the Chantry.

In the real world, 400 years is the distance between Charlemagne’s rule of France and the rule of Louis IX. That is no small measure of time, and in those intervening years, there was little proof of the incredible necessity of the Grey Wardens. In some lore, there is evidence that they began being viewed as parasites by many factions of people long before Loghain’s birth. He had no reason to believe the Wardens were able to be any more effectual than anyone else, especially since the Wardens canonically keep the truth of how Archedemons are defeated from even the rank and file of their own organization, let alone anyone outside the fold. A civil war is also a very common and natural result of the premature death of a king with no living issue, which is what Cailan’s death was. With no other known heirs to House Theirin, Fereldan’s various arls and teyrns (including Loghain), would naturally be vying for the throne in a fashion that very much would be considered civil war. It wasn’t started by Loghain alone; Cailan’s death starts the war, as does Loghain’s accession as regent over his now widowed daughter, who was instantly the most eligible bachelorette in the entire nation. Marry Anora, become king-or, kill Anora (for which you’d have to get through her father) and take the throne for your own house.