Edit: While some points may remain useful for the sake of reference, this theory is disproved in Chapter 82, and Aberforth's death no longer lacks narrative purpose.

Who killed Narcissa?

Suspects:

• Dumbledore

• Bones

• Lucius

• Voldemort

• Someone else

HJPEV tells us that this doesn't fit the headmaster's style. His style is curiously consistent.

There is one offhand remark, vengeance, and a practical cold-heartedness favoring Bones. "Why not Bones?" is only a little better than no argument at all.

Lucius is presented as a devoted family man. It would be inconsistent characterization for him to do this. That works for real life, but HP&tMoR is fiction, which must make sense.

Voldemort has reason not to do this, as it made a fool out of one of his tools and weakened his side by making them less willing to strike indiscriminately.

I have a 'someone else' theory: Aberforth killed Narcissa. Aberforth is dead, and meaningfully so due to Conservation of Detail. We know little else about him from HP&tMoR. Only that he didn't testify against his brother in the death of his sister, and his brother got quite stern when he died. Basically, this theory allows me to put a piece in a puzzle because it fits, not because the image on the piece makes me think it goes with the pieces next to the hole. Also, I get to write the following paragraph.

In a world where innocents are dying, where evil is winning and good people live in fear for their loved ones, one man had the courage to do what must be done. Aberforth Dumbledore is Narcissa's Immolator.

Aberfoth kills his enemy's wife, informs his brother of what he's done, and then dies either at his own hand or, less style-consistently, his brother's. He knows that his brother will take this atrocity/sacrifice and make the best of it, and in so doing he saved countless 'light side' family members.

He did it all to make up for killing his sister and allowing his brother to kind of take the blame. Maybe.