Narcissa knows she did what was necessary. Her family expected her to marry, and marry well, before bearing and raising the heirs (or heir, as it turned out) of an old pureblood family.
How would it have looked if, on receiving an offer of marriage from handsome, well-to-do Lucius Malfoy, she turned him down in favour of a Mudblood? A Mudblood *witch*, at that?
But still she wonders, as one does. Did *she* drive Lily into James Potter's arms, and to her death?
Is it possible that if she had never ended the affair, Lily Evans would still be alive?