Walina wrote...
I found it strange that it will be hard to convince Alistair to let Loghain die at his place, why ?
1) You can convince him to become king when he doesn't want to
2) You can convince him to sleep with Morrigan
3) You can ask him to join a threesome
4) You can convince him to let Zevran on the team
5) You can kill Isolde or anyone you and he will remain at your side (till he had enough though)
Come on, tell me why should it be harder to ask him let go of Loghain when you just manipulated him earlier ? ^^ Unless, he care more about revenge than the PC ? It will be so unlike him in my opinion 
Then apparently you don't know him very well.
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.