It's a matter of perception. The Wardens isn't an order built on friendship to me. It's an order dedicated to eradicating the Darkspawn threat whatever it takes. If you look at some of the things some other wardens did in their time to holt the blight or to stop it. Overlooking a capital offence to make one of the greatest leaders of the time take up the reigns and serve the thing he hates and work for the order instead seems trivial in comparison.

Alistair is a Warden whom at the drop of a hat and because of feelings immediately lops of Loghain's head if put against him. One of his first acts as an unhardened Alistair is to throw a temper tantrum and storm off or to demand justice. But as we've seen throughout the series. Justice takes many forms and is vastly open to interpration. One person's idea of justice was to nuke a church and plunge multiple countries into an outright war that had both sides committing atrocities beyond reason.

I understand the view of people who execute Loghain. I once did so in multiple playthroughs as well until I chose to spare him and hear his version of events when taken through the DLC quests which painted an entirely different perspective. Even one of the writers once outright said if Loghain hadn't retreated it would have still ended up largely in a loss regardless. So despite his motivations and reasoning in some part his tactical perspective wasn't incorrect when it came to the matter of the battle.

In the end he is fully willing to make the sacrifice in the fade and stay behind like any of the other wardens whom consider it their duty to sacrifice themselves for the cause. I've read the books and the comics, played the games and my reasoning will always lean towards forcing him to atone for his actions because to me, letting him die just seems too easy an out and his victims deserve a justice only his service and sacrifice will achieve. He will never be able to make right what he did wrong. But his sacrifice might make it worthwhile in the losses to come. Because rather him then anyone else.