For me personally, there is really no excuse for what Loghain did in and after ostagar. but if you try to understand the reasons behind his action, you'll see that he's really not the power hungry person he seemed to be. I think what prompted him to retreat his forces from ostagar was first, as a general he saw that there will be no victory in that battle so he saved his troops. and second, he hated cailan after cailan taunted him that they would bring in Orlesian forces to help them against the blight. If you've read the book, the stolen throne, you would know that a young Loghain was forced to watch his mother be raped by an Orlesian commander, and his father died trying to buy Maric some time to escape the orlesian. So just from that you'll understand how much Loghain despises the Orlesian, and to be taunted by your King and also your son-in-law that he'll bring orlesian forces to the country you liberated from them, i think that was the last straw for him.

and another thing, i suggest watching the banters between wynne and loghain when you bring him back to ostagar, there's a video of it on yt.