The man who spends the whole game whining to anyone who'll listen about how he was "enslaved" and oppressed by Orlesians has no qualms about forcing his regency on the unwilling Ferelden populace and selling his own citizens into actual slavery to fund his regency. Nothing about his backstory will ever make me feel sorry for him or excuse him after that.
Part of me wished we could talk Alistair into accepting Loghain, but without knowing Log's backstory, it wouldn't make any sense. He was blinded by hatred, not mind controlled by blood magic.
Why would Alistair accept Loghain? He betrayed and killed closest thing to a family Alistair ever had, hunted him like an animal for months, and proved to care about his own power more than his own people. He willingly ignored the darkspawn in favor of fighting the nobles to force them to kneel at his feet, and kept ignoring everyone under the sun telling him the darkspawn was the true threat until it was almost too late.
"He was blinded by hatred" is not a valid excuse in my book since he almost got everyone killed for it. What's worse, he ignored everyone warning him that this wouldn't end well to do what he knew wasn't working. "Oh, he has a sad backstory" is no excuse in my book. So what? This is Thedas. Everyone has a sad backstory, but they don't all enslave, oppress, and willfully ignore advice they don't want to hear and get people killed for it.
But I feel like we do see Loghain can have sense talked into him over time.
What game have you been playing? Loghain spends the entire game shutting his eyes and ears to the reality of darkspawn and willfully ignoring the advice of everyone around him who tries to tell him the darkspawn are the real threat and he should put aside his Orlesian witch-hunting to deal with them first.
He ignores Duncan and the Grey Wardens (darkspawn experts) telling him it's a Blight because obviously he knows better. He ignores reports of the darkspawn flooding in from the Kocari Wilds because obviously the imaginary Chivalier army trying to cross in from Orlais is the bigger threat; post soldiers on that border instead of the southern border. (I'm sure the darkspawn horde will wait patiently for him to be done forcing the nobles into line and hunting every last imaginary "Chevalier" before raping, pillaging, and burning their way through the Ferelden countryside.)
He ignores his daughter Anora's political advice in favor of Howe's, even though she's been acting queen for five years and he is a minor noble who just butchered the most popular and powerful noble family in broad daylight. We see in cutscenes over the course of the year he ignores Anora warning him that they should deal with the darkspawn first (Anora: "Should we not be fighting the darkspawn instead of each other?" Loghain: "The nobles need to be brought to line, and then the darkspawn"), and ignores both Anora and Howe warning him that his forces are stretched too thin and that he can't fight the civil war and combat the darkspawn (and guard their Ferelden/Orlesian border, apparently) at the same time. (Howe: "We simply do not have the manpower..." Anora: "We need help, father!" Loghain: "Ferelden will stand on its own two feet!") Even right after Eamon calls a Landsmeet, if the Warden offers to join forces against the darkspawn instead of wasting more time fighting each other and dragging out this pointless civil war, Loghain rebukes you and insists on continuing to prolonge the civil war at the expense of dealing with the darkspawn.
Loghain is an idiot. It takes the Warden literally beating sense into him with a weapon (apparently the only logic Loghain can understand) for him to see reason, but by that point it's far too little too late. By that point the South had fallen to the darkspawn thanks to Loghain's idiocy and neglect, and it takes the combined forces of Loghain's/Howe's forces, the Bannorn's forces, Eamon's forces, and all the Grey Warden treaties to just barely save the North (and Denerim) from getting swallowed by the Blight. If Loghain had his way, he would have continued to ignore every warning under the sun (including Anora and Howe and the Bannorn trying to talk sense into him) and waited until Ferelden was swallowed by the Blight before even considering, "Maybe this wasn't the best course of action."
Loghain constantly ignores all attempts to talk sense into him time and time and time again over the entire game. He cannot be reasoned with, he cannot have sense talked into him. He can only be beaten martially, and since he hid behind his walls and armies most of the game the Warden can't get to him till nearly a year later, and by that point it was almost too late.
Loghain's an idiot. I think Alistair was right to reject him, as is any Warden who said, "No, I'm not getting my hands on that train wreck."