Disclaimer: I do not own Harry Potter.
Narcissa was not born to wield power.
Narcissa is sweet mouthed, honey tongued, pretty faced. She was born to bear child, created to serve as ornament (loving wife and mother.) She was made to be quiet, to close her lips and part them only to sip at her tea or to sing.
Narcissa was born to be a doll.
She was born a doll, but she will not die a doll, even if such is unbeknownst to man and wizard.
For now, she holds the world in her mouth and life in her breath, fate in her eyes.
In this moment, she is God, for she holds everything in the cast of her lips. She is God because she could bite down upon the world and crush it in her jaws. She is God because she could blink the fate away as simply as she might blink dust from her eyes. She is God because she could turn that life in her breath to death.
She will not.
She does not do this for Harry. She does not do this for the good of all that lives.
If there is a reason why she does not bite to shatter the earth right now, it is her son, her one true purpose in life - and if she saves the world now, she saves it for her son.