Nightmare Fuel / The Curse of M

• The entire premise, really. Imagine contracting a terrible fever, then one day waking up to find yourself saddled with a magical ability over which you have no control whatsoever. Some countries execute the Cursed when they find them. What America and Canada do is only marginally better: whenever one of the Cursed is found, they're shipped off to the Institute, a place so isolated the only way in and out is by air, run by a man with the ability and willingness to sift through your every thought with little to no effort.
• Geezer's memories of Vietnam, and how he suffered such horrible burns to his hands. The fact that Von Rached is so dispassionate about torturing him with them just makes it that much worse.
• Von Rached himself is pure Nightmare Fuel. He's managed to extend his lifespan to an unnatural degree, and is such a powerful telepath that nobody has any defenses at all against him. He doesn't do things For the Evulz; he's first and foremost a scientist who regards his patients as nothing more than lab rats. He toppled Nazi Germany because it annoyed him, and then went to America, where he's been allowed to work unhindered for decades.
• Wrigley. Though it's only mentioned in passing, he almost burned his girlfriend to death when he woke up with pyrokinesis.
• Von Rached's rape of Lorna. It's entirely offscreen, but the Nothing Is Scarier aspect is arguably more disturbing than if it had been shown. She wakes up with no memory of what happened to her, but the type and extent of her injuries tell a story on their own, and leaves you wondering just what the hell he actually did. It's nearly enough to drive her to suicide, and the psychological and emotional damage it deals her follows her right through the end of the book.