So I was perusing the Bioware social site and I came across this thread
http://social.bioware.com/forum/1/topic/47/index/202678 which had been graced by lead writer David Gaider. For those of you who don't lurk around the social site (and/or don't feel like mucking through 10+ pages of posts to find this) I present to you a bit of his insight on Alistair's character and motives. He explains (or tries to... some were just not having it, lol) why Alistair could never let Loghain live, among other things. I found it interesting to get the "insider" perspective, as it were. There are certainly **spoilers** ahead.
"Personally I don't think Alistair would let Loghain go. He swore that Loghain would die for what he did, and while there are many things that Alistair can be talked out of, things that he sees as his duty or promises he's made are not those things. I don't really mind if someone wishes to make a mod and pretend that Alistair would do that, if they really really must have their happy romantic ending prancing off into the sunset and nothing else in the game is important to them, but it *is* pretend. So long as you're okay with that, so am I.
As for Riordan, this has been discussed elsewhere. He didn't know that Alistair and the PC weren't aware of the Archdemon Solution (not realizing that they were both new recruits, perhaps). Even if he did, there's also no way that Riordan could have realized that the showdown with the Archdemon would come before the Orlesian Grey Wardens could be brought to Ferelden. His suggestion that more Grey Wardens might be useful is just that.
Just because YOU guys know now in meta-gamey fashion that making Loghain a Grey Warden is the more convenient way for the romance plot to go your way doesn't mean that Riordan or Alistair know that. Frankly, even if Alistair DID know I'm not sure that he would believe giving Loghain that honor -- remember, he considers being a Grey Warden and making their sacrifice an honor -- to be a good thing." --from page 5
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"You can convince him to become King because him not wanting to be a King is based entirely on his self-esteem. He doesn't think he would be a good King. He doesn't think anyone would want him there. Is it surprising that someone he loves telling him he should be King after all might be something he'd consider?
You can convince him to sleep with Morrigan. Yes, but this has nothing to do with contravening his duty. He has no duty to NOT sleep with Morrigan. He never promised not to give her a baby, and while he doesn't like her she's never DONE anything to him. Chances are, even if you bring up the baby he'll only agree if he knows the ritual will save your life. Or SOMEONE'S life. He's not even concerned about his own.
You can convince him to join a threesome. Again, he's not celibate, or even a prude really. Twisting someone's rubber arm is not exactly a giant leap for his character.
You can convince him to let Zevran join the team. And why couldn't you? Zevran attacked you all, yes, but he didn't kill anyone in the party. If Zevran had, do you think Alistair would have been so eager? Hardly. He doesn't trust Zevran, but it's not as if Zevran killed Duncan or any of his friends.
He will remain at your side after killing Isolde or Connor, yes, but keep in mind the difference here: at some level he feels he owes Arl Eamon and is angry about being directly (or indirectly) responsible for the death of his wife or son -- but their deaths break no promise of his. It's not as if he didn't know something needed to be done in Redcliffe, he's simply very upset how it went down.
With Loghain, however, it's a different matter. He swore that the man would die. Loghain is responsible for the death of Duncan and all his friends in the Grey Wardens. He thinks the man is evil, and if anything his DUTY would be to keep him OUT of the Grey Wardens.
You think Alistair could be convinced because you want to think that. Alistair isn't spineless, but you're arguing that he is and should bend to your every whim because-- why? Because you're selfish? I get the desire behind it, but let's not mistake one thing for the other. Anyone saying they "know Alistair" and believe he would actually do such a thing is ignoring one of the most important things about him.
If you really want to change Alistair into someone else because that makes it more convenient to have your happy ending, I get it and I understand. But I know my character." --from page 6