Is he there because Bioware need an extra npc to kill or because Alistair is so narrow minded that blamed Loghain for 100% of wrong in the world?
Howe had no problems in torturing people for fun regardless of their status and race or to hire mercenary to kill the Grey wardens...
Oh Alistair facepalm....
Alistair wouldn't have gone to the Denerim Estate to rescue Anora in the first place. So it would all come down to the fight breaking out and then the duel. After which, Howe would swear that it was all Loghain's doing and he would promise the utmost support to Alistair's reign. I think DC also states that Alistair's marriage to Anora was a compromise to break the deadlock at the Landsmeet. It also suggests that Alistair and Leliana had a relationship. And Morrigan is with him in the final battle, which means he did the dirty with the swamp witch.
Isn't he superior to the your Grey Warden though?
Well certainly not if he quits. But no, he was basically the same rank. He said he was recruited about six months before you. Also, if you're a human noble, then I would say your rank is naturally higher than his. But he follows the Warden regardless.
He's the older recruit, so he didn't disobey anyone - he just choice not to listen to your character and leave (becuase he doesn't care what your character thinks or what not).
He disobeys the Grey Warden Order and deserts. And if you're looking for a superior officer, then Riordan is your guy.
Otherwise, yah your correct - I would like to note that if Alistair had been both patient and allowed to, i would've killed Loghian by throwing him to the Archdemon, but eh, Impatience and 'justice for all!' is Alistair's weakness though.
Too bad there wasn't a "Just trust me" option where Alistair would stay in Denerim, even if he leaves the party. He should have at least stayed to defend the people, including Goldanna, like he promised.