After HBP, there seems to be a lot of ideas that Narcissa is actually on the "good side" and that she’s a spy or working undercover, etc. Frankly, this drives me up the wall, because the only reason that people think that is because she showed politeness to Snape and concern for her son in Spinner's End. Having love for your children and good manners does not mean that you sympathize with Dumbledore's cause. Looking at the depth so many of JKR's characters have, I think it's sort of insulting to her character design skills to say Narcissa is secretly a "good guy".
3-dimensional bad guy > Cliché good guy.
Of course, I don't think Narcissa supports Voldemort either. I think it's pretty obvious that she knows her life would be a hell of a lot better if Voldemort were wiped off the face of the Earth. But Good, Secretly Unprejudiced!Narcissa really hasn't got any canon backing. How do you explain her actions in Madam Malkins, what with no zomg oppressive husband there to put up a front for? That is how that theory goes, isn't it? And why does she not correct her darling angel's bigoted comments, since she would naturally want him to not believe such things? Why respond to Harry's insult of her husband with a comment that's one step away from "I hope you die"? Or no, let me guess, she actually meant that Sirius is alive and will be reunited with Harry before she is reunited with her zomg oppressive husband? Oooh, so cryptic!
And then she has to add in that completely random little comment about Hermione, who's only distress she's caused Narcissa is having muggle parents. Who exactly is Narcissa putting on a show for here? The good!Narcissa theory's justification of Madam Malkins is so roundabout and contradictory, why take it at anything other than face value? Narcissa insults those who are not like herself, just as Lucius did in CoS. Trying to justify Narcissa doesn't make much more sense than trying to justify Lucius.
And people think Draco is a bully because of "the insurmountable amount of pressure from his father" or cane abuse or whatever the hell people say (in all fairness, that's what I thought (or wanted to believe) before, but then I went back and actually read the Lucius and Draco scenes again...). Please. Both of his parents have blatantly and proudly insulted others because of their status, beliefs, and bloodlines in front of their son, right there in the sweet black and white of canon. And I'd be more than willing to bet that instances like that aren't exactly rare. But I digress; Draco's specific bullying influences is an essay for another time. All I have to say on that subject is this: Monkey see, monkey do.
Anyway, back to the zomg oppressive husband part. Narcissa was in Slytherin, was she not? From what I've gathered, the theory goes that Narcissa was forced to marry Lucius and only stays with him out of concern for her son, correct? Other than there being no evidence at all in canon for this, it's also about as un-Slytherin as you can get. From what I've seen in canon, comfort is right up there with power on a Slytherin list of things they want most. Sacrificing herself to preseve family honor and purity, sacrificing years of happiness and freedom because she thinks her son should have a zomg oppressive father.. way, way too selfless and un-crafty and just plain uncomfortable to be in character for a Slytherin. If she didn't want to be with Lucius, I'm pretty sure she would have gone the way of Andromeda.
There's also the Kreacher incident in OotP. Why was Harry so sure that Sirius was at the DoM and the final reason he made the decision to go? Because Kreacher told him so, because Narcissa told Kreacher to do so. When Dumbledore talks about Kreacher's orders, he either says it was 'The Malfoys' or 'Narcissa' who was told by the Dark Lord. There's no kind of "Lucius told Narcissa to do this or that because Voldemort told him.." or anything. I mean, Dumbledore seems to know little things like that too, you know? I think Narcissa gladly gathered information that would help her husband succeed in his assignment.
Lastly, we have the Black Sister Formula. JKR likes them formulas, eh? I think the Black Sisters’ characterizations definitely have a pattern of some sort… one that my friend actually theorized on as I was first getting into fandom, a year before HBP came out. This theory is pretty much the main reason the caring, motherly Narcissa in Spinner's End came as no huge surprise to me. Anyway, I think the Black Sister Formula is quite possibly the best evidence for neither a totally good or bad Narcissa.
Basically, on one end of the spectrum, we have Bellatrix, who is obviously for Voldemort and purification of the wizarding race etc. Then we have Andromeda at the other end of the spectrum, who I'm going to say probably supports Dumbledore's side and protection of muggleborns and halfbloods considering her family is composed of them. And then we have Narcissa, who is right in the middle.
Narcissa is prejudiced against impure blood and doesn't support Dumbledore, but she'd also tell Voldemort to go to hell too if that wouldn't result in immediate death. The only thing she cares about is herself and her family. Now, what is the word I'm looking for... what is it called when the only things you care about center around you and yourself..? Oh yeah, NARCISSISM. Well I'll be! That's leik, a pun er sumthin'! Narcissa doesn't favor either Dumbledore's side or Voldemort's because that disrupts the Black Sister Formula. She favors the Malfoys.
Granted, I guess wouldn't be too surprised if Narcissa were to do something that would help Voldemort's demise come a little quicker, especially if she could bargin her help with the good guys in exchange for some sort of reprive of punishment for her son or husband. But going over to the good side? Even being mildly nice to any of Harry's crew? Not a chance.