I also recommend saving Loghain at least once for further details, but this does not alleviate his responsibility for allowing Arl Howe to perform such atrocities, and listening to his vile advice.
That begs the question whether Loghain actually knew about what Howe was doing in Denerim because Loghain was out with his army pretty much the whole time, if you listen to the gossipers and the rumors from Bodahn or inn keepers. And the only bit of advice I saw Howe give Loghain in-game that Loghain followed up on was the use of the Antivan Crows.
There is a rather distinct lack of in-game proof that Loghain even knew about Howe's atrocities. Bring it up at the Landsmeet and he rightly brings up that Howe's actions are his own and cannot be pinned on him. Not without proof that he was complicit or knew of it.
And TEWR brought up a good point about how Howe (lol, unintentional pun) controlled the entire north of Ferelden, its supply routes (even from Kirkwall as we find out in DA2), had a large force of arms, and opposing Howe would mean fighting a war on two fronts and being completely surrounded.
For me, Loghain is guilty of many things, selling elves into slavery, hiring a blood mage to poison another noble and hiring the Antivan Crows to assassinate the protagonist. When I discuss Loghain with people, and they want him executed for these things, I can see where they come from and why and won't even try debating it because I think people who are slavers should be punished to the full extent of the law no matter their position, and despite what the developers said about the poison being non-lethal for Eamon, if someone just plays the game and didn't pay attention to the devs comments they would have no idea about that. But when it comes to Ostagar, how much he knew about Howe's dealings, or his alliance with Uldred, that's where I think things turn into shades of grey.
On Ostagar itself, I agree with his assessment of the battle. It could not be won, and he tried to save as many men as he could so he could continue the fight another day. He then tried to rebuild the army as quickly as possible and take the fight back to the darkspawn, but his lack of political tact and his abrasive personality threw off the nobles, not to mention Teagan's well-intentioned but horrible use (and understanding of Loghain's motivations) helped incite other nobles to turn against him, not to mention the nobles who wanted to take advantage of the power vacuum, and I blame the other nobles more than I blame Loghain, without withholding his own fault in that political screw up for the civil war itself.
On the issue of his alliance with Howe, and Howe's actions, I hold Howe to be far more responsible for what was going on than Loghain. Howe played Loghain like a fiddle, seized so much power that it became disadvantageous to oppose him, and since Loghain was out with the army, he couldn't pay attention to all the little details that Howe was doing. He was likely focusing more on troop movements, tactics, and defeating those who opposed him (the idealistic nobles) or Anora (the ones taking advantage of the power vacuum).
Although to be fair, Loghain has less credibility if the Warden is a Cousland who outright tells him of Howe's treachery, most of my Wardens are not, so I don't see how Loghain would know about Howe beyond him being unpleasant, as per Howe's own personal codex.
As for his alliance with Uldred, I don't blame Loghain one bit for how things turned out at the Circle. He wasn't even involved with what happened. He offered an alliance with Uldred, more autonomy to the Circle if the Circle cooperated, Wynne came back and told Irving about Loghain's retreat, and Uldred panicked when the other mages weren't going along with what he wanted and led a revolution on his own, and ended up becoming an abomination in his panic, which is what Nial says happens if you talk to him more than you have to in the Fade sequence.
That is my personal take on Loghain. He is guilty of some things, and if you roleplay a Warden, or as a gamer oppose those things and feel he deserves death, more power to you. But it is also my opinion that blaming him for things he wasn't part of, and to the (occasional) posters who take defending his retreat at Ostagar to also mean defending everything about him is just taking their dislike of him way overboard.
I also don't see how joining the Wardens is the honor Alistair believes it is. I mean, sure I can see why people would see it that way, and would see Wardens as heroes, but Warden are also ruthless and do pretty extreme things in the name of stopping the blight, which is something even Alistair mentions if you talk to him at Ostagar before returning to Duncan to go into the Wilds.
This post was a bit longer than what I initially intended, but that's how I see things.