There are a lot of reasons to want Loghain dead. You (or your warden) feel personally betrayed by what happened at Ostagar, they hold him responsible for slaving, poisoning Eamon, hiring Jowan and using a blood mage to do it, working with Howe, whatever, it's just a game and everyone can have a different view or playthrough. Some people will kill him in one playthrough, then spare him and have him sacrifice himself in another, or spare him and do the Dark Ritual so both the Warden and Loghain lives in another playthrough. Some people may only be willing to do one of the above, and as it is subjective, no one would be wrong on how they want to play their game.
With that said, my personal view is that if people want Loghain executed, it shouldn't be because of Ostagar alone. There's enough evidence that lends credence to what Loghain has been saying the whole time, they couldn't win the battle and Cailan brought his death upon himself. Saying he planned it goes against all evidence, including the lead writer David Gaider's own comments. If you feel slavery deserves death, which my Hawke did in DA2, killed all slavers he found, then Loghain who willingly sells the elves of the alienage to Tevinter as slaves is guilty of such a crime, but if you feel forcing him to be a Warden and serving in Orlais, the country that he hates above all else is worthy enough punishment, and being a Warden is not really an honor at all despite Alistair's rose-colored perspective without Duncan, then you condemn Loghain to a life working for the good of all man, elven and dwarven kind in a country he absolutey hates, where he's likely to die a gruesome death, then again, by all means. Or you may feel like he deserves the chance for redemption and allow him to die killing the archdemon.
There are no wrong reasons, but I feel that whatever our reasons may be, they should be backed by legitimate facts. And yes, I know emotions aren't governed by logic, but it's not defending the man's honor and everything he did when it's pointed out that Ostagar was more Cailan's fault than Loghain's, and the Horde being so much larger than they expected.