[TteFAboB]

If you accept the possibility of the murder to be considered diminishing then any extension of the murder itself is an extension of the diminution. If this project would be more or less diminutal than the act of murder itself is arguable, but the scale is not exponential: being diminutive even by the smallest degree would already be enough to add to or even increase the original diminuent level pushing the total amount of diminution higher.

One part can certanly diminish the whole as happened on the day of the murder, two less individuals in a strict mathematical sense. Of course, humanity has not shrunk by two, maybe this is where the confusion comes from: the final net amount has not been diminished at all! The outcome is positive, but diminutively reduced. Humanity as the quality of all members of the human race can also be diminished by a single person considering this person wouldn't have been able to do any diminishing if it weren't for the inaction of everybody else for example. But that was not the case here. There are more people involved. I do not posses the omniscience required to tell you precisely to what extent the net result of Humanity as a quality would've been affected by this case but I suppose that as Dave Grossman, author of "On Killing", says it definitely wouldn't be good to further desensitise people towards killing.