(In response to your "what we should be talking about" tag, I've got a question for you): what do you think about the speech Armand gave to Madeleine? Do you agree to it? Do you think all the vampires we know are troubled by such thoughts?
I think the best answer to this is actually where they seem to stand on humanity.
Armand is a monster who is clinging to what it means to be human. Which makes sense, to me, considering how much has been ripped away from him without his consent. The world is not as he knows it; he lost his home so many times, and he will never be able to return to see it again. He knows she can't quite fathom what that means, but to be fair, neither can he.
When he speaks of killing a person each night, what do you think of the children? The mothers and fathers? He's asking her if she can see them as people and still take them away. Considering Armand sees the world in gradations of evil, this is something that matters to him. He may not see humans as much more than fodder, but he has been one once, and that does trickle into his decisions.
Madeline, on the other hand, is a human who is comfortable with monstrosity. She's a war-child. She has found comfort and humanity in a monster's arms before, this will be no different. She has been seen as a monster in her community, and despite that she refuses to back down and hide. It is a point of pride, for her.
She needs to eat food to survive. And if humans are food, then that is what she will eat. She's pragmatic on the fact of survival; she has little of humanity to cling to other than what she can carve out herself (carving the apple).
And Armand has respect for that. Assad has mentioned in an interview (don't remember which one, sorry!) that he did like Madeleine, and found her interesting. She is not weighed down by the guilt of what it means to survive.
So perhaps we can see this in terms of that: Armand has survivors guilt (probably connected to the piazza/Marius) while Madeleine has none.
