That doesn't really invalidate what I said, just makes Alistair more immature. And if you accept Loghain's surrender as soon as he yields (before making him a Warden is even suggested) Alistair still gets angry.
I kind of lost respect for Alistair at that point.
I mean he stands by you when you can do some pretty horrible things, for example: leaving the people of Redcliffe to be slaughtered by zombies, killing the arl's wife or son, defiling a holy relic, betraying and killing your friends, exterminating a Dalish clan and allowing alienage elves to be murdered by a blood mage.
But NOT killing an unarmed opponent who has surrendered (in front of his daughter) is where he draws the line. I mean I know he has a valid reason to hate Loghain but that reaction seems....petty. (btw yes I do have a similar issue with Sebastian but I can sympathise with him a tiny bit more as unlike Loghain there isn't really a proper way to non-lethally punish Anders).
But basically it doesn't really matter, their paths diverge and the both move on with at least somewhat content lives.