First off let me say this.
I've been in En World for about a month now... and up until now this is the most interesting thread...
Without any intention to undermine the OP or the rest of the posters, i believe that the discussion between Celebrim and Viktyr Korimir is one every DM should read and break his head over...
Not that a DM should be a person that advocates his own "morality" through his game....
...but through the game... through the story... players and DM alike should really put their very personal beliefs to the test.
D&D sure is fun for killing monsters, for it's intrigue, for solving riddles... but if it can provide for something more to think over... it's even better.
I totally agree with how Celbrim keeps things in the grey area... without putting neon signs over "good" or "evil"... for that's how real life is as well.
And while i agree that D&D isn't or shoudn't be a simulation of real life (in general...), IMO the ambiguity of morality... the repercussions of actions... should be simulated to the full, for not only is the game more interesting and intriguing, but because there is a small chance that we all learn something out of the experience... about ourselves and about the people around us.
I'm NOT saying that the game SHOULD be played like that, for there are many different ways to play the game and still have fun (simply... killing monsters can be equally as fun, and there's nothing wrong about it). I'm saying that if one wants to employ "morality" in his game... he better go all the way because there is no middle ground.
Enough with saying how cool you guys are...

... now let me be more specific...
At some point... Celebrim argued how there was a possibility that the cultist woman could have helped further with the party's investigation... if she had not been killed that is.
Well... this was a major opportunity for the party to do some more "good" but they have unfortunately missed it. Now, if for example they are late in saving some more peasants, because of the fact that they killed the cultist without interrogating her... well... shouldn't that weight heavily of somebody's conscience? A god... or a third party of lesser "divinity"... seeing things from a distance (or above if you may), deprived of instincts of revenge, is able to "judge" how the act of killing the cultist, no matter her actions or beliefs, was a "bad move" for someone who intends to do "good". In real life this god is within our heads, its our conscience, which... while it is overshadowed when our instincts emerge (and in this case, the instinct of killing the "evil" cultist) it reemerges when our instincts subside. And what defines the difference between good or bad is how one lets his conscience come in the way of his instincts. The fact that the cultist was killed by a neutral character makes no difference whatsoever. A good aligned character's conscience is going to reemerge sooner or later... in his sleep he's going to have nightmares, for the very fact that the cultist "could" have had vital information that "could" have saved more lives, is going to steal his sleep.
As i said above, in real life, this god lives in our heads. And while in D&D this god is a being on its own right, this does not diminish the fact that, symbolically, he is still an archetype of our codes of morality.
Now it is entirely up to the DM on how this influence is resolved.... is it "guilt" in the form of nightmares? is it a "direct" interference from the God itself? ...or perhaps some sort of "indirect" interference?
Is it going to be just a roleplaying occasion?... or is it gonna affect the character's "numbers" in some way so as to mediate the impact of one's actions... this of course is up to each DM's style of storytelling... and up to debate...
I apologize for my english. I hope i made some sense.