Short answer: Because the macrobes didn't care about people, not the people at N.I.C.E. or anywhere else, they only cared about their own designs and ends.
I would add to the first (earliest) answer, that the scene that closes the narrative to the reader on N.I.C.E. does, and likely is intended to, resemble one of the most historically denigrating things in humanity (in the context of Western Reformation thought, at any rate); primitive tribal totem worship. What Lewis portrays is the pinnacle of 'human achievement' coming up through the top of intellection and emerging back out of the bottom in bleak savagery. Whether it was intentional or conscious on Lewis' part or not, it is a coarse depiction of Nietzschean philosophy (recall that Nietzsche himself was committed to a sanitorium), a sort of intellectual Ouroboros where the desire to 'improve on' humanity ends up dehumanizing it completely.
Literarily, there is an underpinning for this in Lewis' nonfiction work "The Abolition of Man," in which Lewis explicitly declares it to be about formal education and morality. In speculating about the group he labels "The Conditioners", he states that
I think they would be inclined to hate the conditioned, for they still
enjoy the comforts of an 'illusionary' morality from which they have
freed themselves in order to direct their fate
(from memory, not an exact quote). Here, as the earlier answer elucidates, 'The Conditioners' would be the macrobes, and the conditioned their acolytes within N.I.C.E., ultimately to be the people of Great Britain. So for the conditioners, their designs having been frustrated by the sane and sanguine 'spirits' opposing them, would have no reason to not vent their frustrations upon creatures they treated as mere tools (rather than allies or valued servants as with St. Annes, as the earlier answer astutely points out). Lewis being a Christian at this point, one might connect this observation to the several New Testament parables using servants as characters and/or object lessons.
Historically, he was one of many post-facto witnesses to the inhumane brutalities perpetrated by the NAZI party and the Third Reich on greater Europe, and having been a veteran of WWI, was sufficiently composed and qualified to actually contemplate and analyze it in a sort of 'intellectual autopsy', if you will. Lewis has also commented on this among his essays, one in particular where he coined the term "bulverism" out of his own personal experience and 'headcanon', if you will. He conveyed at one point in "The Screwtape Letters" Ch. I, I believe, that in his conception there were higher powers hostile to humanity, which wanted to enfeeble rather than enhance human intellect, penning
...that nonsense in the intellect should reinforce corruption in the
Elsewhere (I believe in Mere Christianity, pt. 4), he comments on the progressive inhumaneness of NAZI perspective and conduct, saying something close to
In the beginning, the NAZIs mistreated the Jews because they hated
them; towards the end, they hated them because they had mistreated
So, in a sense, the Macrobes turn out to be as sinisters as, or even more so than, amoral deceptive aliens who are only using humans as a means to other ends (this is a major plot element of the Sci-Fi TV series "Earth: Final Conflict", directed by William Shatner and novelized if I am not mistaken), and only value them insofar as they are useful towards those ends. Recall that elsewhere in the novel and the Space Trilogy, Earth (Thulcandra) is portrayed as a world under a sort of seige, and the powers collected there are striving against an immensely superior power, the way that Jerusalem strove against Babylon or Carthage sought to rival Rome. So there is a sense in which the Macrobes are intensely desperate to make any kind of progress, and setbacks might be treated with a sort of supernatural manic response as the scene portrays. To stay within literary canon, juxtapose what happened with Devine et al. and 'the head' at N.I.C.E., with what happened with Weston on Perelandra in the same-titled book; treat Weston as a living subject for the Macrobes (the human name for them, in "Out of the Silent Planet" they end up being called the " 'bent' eldil") and you will get a better perspective on their motivations and methods.
The parallels to tribal totem worship, while apparent to me in the scene, are better treated by someone whose specialty is anthropology, not philosophy or literature.