This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Sacred And The Profane
The following passage is a sample of the madness.
"I'll give you ten seconds," said Zirah. His eyes were lit with bright, childlike delight.
He leaned towards Hastur, who was rapidly backing away from the enthrallment on Zirah's face.
Ligur had forgotten about stealth and was fleeing, crashing through bushes and tripping over
"Run," Zirah whispered. Someone yelled behind him. None of them noticed.
The figures of the two demons blurred and vanished.
The problem with this Earth, Zirah thought, was that nobody ever let him relax.
He followed them.
Reading this immediately after Good Omens itself is extraordinarily disturbing, because you know that this isn't the way things should go; please God make it stop, but it doesn't stop and in this story, this is the way it goes.
Most painful might be the dichotomy between Aziraphale, who insists on helping people he has to hypnotize, and Zirah, who happily kills babies.
Zirah is completely insane. Scarily so, he almost kills a child for no other reason than the baby is crying, and he wants the baby to stop, and there is a heavy dictionary closer than a bottle of milk. He sees nothing wrong with the attempted murder, and reacts with genuine surprise when he is stopped.