This is another missing exception: if the act that killed Narcissa, despite her having no dirty hands, saved more lives than it took. (Whether or not that applies in this case, it would certainly be Dumbledore's defence, and Harry might buy it, given enough evidence.)

It seems likely that he would give it at least some weight. The justification reasoning even crossed his mind when considering the possibility that Dumbledore set Voldemort on him and his parents.