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Nightmare Fuel: All-Star Superman
Sure, Lois was suffering chemical paranoia at the time, but the events are scary when you think about it—especially Superman's stalker-esque super-speed entrance with the flowers while she is alone in the fortress surrounded by terminator faced robots. Also, right after, as Superman is explaining things, it's still pretty scary. When did he get Lois's measurements? He memorized her genetic code? Think about that a moment—a man with the power of a god has secretly got your measurements and memorized every line of your genetic code without your knowledge?
The Bizarro invasion. Imagine a cube shaped world that eats other worlds, that emerges from a dimension beneath ours. The Bizarros themselves are especially creepy. Hordes of misshapen zombie-like duplicates screaming backwards nonsense, and when they touch someone, that person loses their face and BECOMES a Bizarro.
Steve Lombard (who's immune to them due to his steroid habit), a character who's largely a complete jerk, freaks out during the invasion, particularly when he has to toss a friend of his off the fifth floor of a building in self-defense after she turns. "Good God, I just threw her out a window..."
Bizarro-Green Lantern's ring makes everything he doesn't think about real, except he can only think of everything. Isn't that terrifying, paradoxical, sad and mindblowing on so many levels at the same time?
One can get the impression that Bizarro-Green Lantern meant that he was too stupid to think of anything for his power ring to work. Your typical Bizarro speech is often the opposite of what the pale doppelgangers really mean to say, and their tendency to engage in Confusing Multiple Negatives can sometimes make it hard to understand exactly what they mean.
Immediately frightening, but even more so when you consider the implication: That Luthor can and does grow people who believe that exploding is their basic human right, and uses them just to accuse Superman of interfering with their rights if he stops them from killing themselves. A moral conundrum which even Superman could in practice only solve by helping the thing die.
Parasite in the animated adaptation. Even though there's no gore, his murder of guards is just chilling.