Because Narcissa is pretty awesome, and Voldemort (although awesome) was never vey bright.
Dissclaimer: I own nothing.
Judas
Of all the people he thought could betray him, he wold have never expected this.
He should have known, really, because it had always been there. He never saw it, but he sees it now.
It's too late, but he sees it now.
.
Narcissa never took the Dark Mark.
Yes, she is Death Eater's wife and she is Death Eater's mother, but she was never, and will never be a Death Eater herself.
She is a Black, among everything, and Blacks serve no one.
Not even the Dark Lord.
"You're disgracing your family" Bella said to her, the day she refused to take the mark.
"No, you are."
Bella never understood. No one understood. But Narcissa always believed. She was the truest believer of them all.
She believed in blood purity, she believed in the power of wizards over muggles, she believed in the elimination of mudbloods, but, what she believed in the most, was the power of the blood.
She was a Black though and though, and, when the time came, what she cared the most was always her blood.
Nothing was more important, nothing was truer, nothing was strongest.
She held her head high and her priorities in the open, but people only see what they want to see. And she never led them to believe anything else.
.
She would never look at him.
Not in fear, like the others, but in pride.
It should enrage him, the fact that she thinks she is better than him, that she thinks she is above him even when her whole family is serving him. But, surprisingly, it doesn't. He finds her entrancing, the way she keeps her cool when all the other shudder in fear at his mere presence, the way she stands tall, the way she looks at the others, like they were a disgrace, the way she carries herself, like she owned the world.
She is the truest believer of them all, he remembers thinking proudly.
And oh, wasn't he right.
.
Narcissa is not in favor of the Dark Lord, as the world assumes.
But she is in favor of honoring her family's name, of fulfilling her duties as pureblood and of keeping her bloodline pure.
The Dark Lord's blood is not pure, like hers, so there's no way she could respect him.
It's as simple as that.
She follows him because her husband follows him, and she is expected, as a pureblood, to stand beside her husband.
It all comes down to the blood for her, in the end. Nothing else matters.
Even if she believes in all that she believes, she doesn't really care about mudbloods, muggles and blood traitors, as long as they don't come close to her. It's obvious to her, that the Dark Lord is just trying to get rid of the only thing that marks him as one of them, as a mudblood.
She wonders sometimes why no one else sees it.
She purses her lips in disgust. They're all like lambs, they know nothing better than to follow.
But she's a Black. And she doesn't care that even the Black's don't understand it.
Narcissa Black follows no one.
.
He should have known.
How was it possible that he didn't see it?
She was not like the others. She was not after the power or the fear or the simple pleasure of causing pain, like her sister. She was there to please him, like the others, only...she was not.
He was supposed to be the strongest wizard of all time.
How is it possible that he didn't see Judas sitting beside him, at his table?
"You examine him, tell me whether he is dead."
How could he be so stupid?
"Is Draco alive? Is he in the castle?
It's the only thing she cares about. Her blood. She never really cared about anything else.
"Yes"
He does not expect her to save him. He's just that kind of good person.
"He is dead."
She doesn't care that he is a blood traitor. She never cared about that.
.
She was the one who betrayed him, in the end. She, who believed more than anyone.
The problem was that what she believed in was so much bigger than him, and Voldemort was too blinded by his power to notice.