Towards the end of the book, following Merlin bringing about the destruction of much of the N.I.C.E., Dr. Filostrato, John Withers, and Reverend Straik go to see the Head. The Macrobes promptly demand another head; Withers and Straik promptly kill Filostrato in order to supply it with one. Eventually, they realize that the Head will just demand another Head, and they promptly turn on each other.
That being said, why did the Macrobes make this demand? Why the gratuitous killing of people that were working for them?