In Larry Niven's The Ringworld Engineers, Louis Wu, Chmeee, and Harkabeeparolyn
travel to the Ringworld's control center on the Map of Mars in the Ringworld's great ocean, in order to find some way of re-centering the giant structure on its Sun.
But there they run into Teela Brown, who
remained behind from Louis and Chmeee's last visit to the Ringworld. She's eaten Tree-of -Life root and become a "protector".
But it's weird:
Protector-Teela tells them that there is a way to save the Ringworld, but doesn't tell them what it is. She's actually come there to try and stop them.
Teela tells Louis:
"Furthermore, there is a solution that does less damage, yet it is too much damage, and I cannot permit it."
Teela's solution will kill 5% of the Ringworld's population to save the other 95%. That's 1,5 trillion sapient humanoids out of 30 trillion. It will be a slow death by radiation.
Knows it has to happen, and drops hints to give Louis a chance to figure it out, she nevertheless makes them fight and kill her in order to gain access to the control room.
Louis and Chmeee's advantage is, as Louis puts it, is that
Teela is fighting to lose. One of the many strange compulsions of Protectors forces her to engage in "doublethink" to avert disaster.
Why can't Teela resort to simple utilitarianism?